There are lots of figures floating around as to the actual financial impact of Brexit on the United Kingdom. They are all extrapolations based upon the impact to GDP and below I will show some of the estimates and methods by which they are calculated using data from the UK Office of Budget Responsibility.

Lets start with the LIE.

The UK government does not currently pay large weekly sums to the EU as it did before Brexit. When it was a member, the net contribution was roughly around £150–£160m per week at most (after rebates and receipts), far below the widely promoted £350m figure.  I will use this exaggeration in the final opinion, because the one thing that is clear from this whole thing is that the liar’s exaggerated but the pro-EU people did not.

The lie was £350m a week was bing sent to the EU, which is 2.33 or 2.18 times more than the actual figure of £150–£160m per week respectively.

Where the £850 m/week idea comes from

• An economist, Tim Leunig, flipped the Office for Budget Responsibility’s estimate that Brexit has reduced UK GDP by around 4 % to 8% compared with staying in the EU, and calculated that that lost growth translates to about £850 million per week relative to a hypothetical “remain” scenario. This was commentary, not an official government statistic. https://www.thelondoneconomic.com/politics/economist-explains-how-the-brexit-campaigns-350m-a-week-claim-was-actually-a-total-bargain-399522/

There is no single definitive weekly “cost”

• Independent estimates have varied widely because it’s inherently difficult to know what the UK economy would have been like without Brexit:

  • Some studies have put it at £500m/week in lost economic activity compared with a remain baseline. https://www.cer.eu/in-the-press/brexit-costing-britain-%C2%A3500m-week-and-rising-says-report

  • Others have suggested around £550m–£800m/week in lost output or lower GDP. https://www.standard.co.uk/news/politics/brexit-has-cost-economy-ps550-million-a-week-since-referendum-report-finds-a4109216.html

  • One think-tank put the economic hit at around £40 billion per year roughly £750 m/week mainly from reduced tax revenue due to slower growth. https://www.bylinesupplement.com/p/brexit-costs-us-750-million-a-week

These cost estimates are all comparisons they assume a “counterfactual” where the UK stayed in the EU and grew differently. That involves assumptions about trade, investment, productivity and prices. Different economic models produce different results, and no single number is universally accepted. Independent analysts and official bodies like the Office for Budget Responsibility stress uncertainty and wide ranges in their forecasts.

OBR’s Central Long-Term Assumption on Brexit’s Effect

The OBR’s official forecasts incorporate a specific assumption about the lasting economic impact of Brexit:

• They assume that the UK’s trade with the EU will be permanently less intense about 15 % lower than if the UK had remained in the EU, due to non-tariff barriers and friction from the Trade and Cooperation Agreement.

• This reduction in trade intensity is assumed to result in the UK’s potential productivity and thus GDP being about 4 % to 8% lower in the long run than it would have been under continued EU membership.

  • Under this assumption, part of that reduction (about 2.7 %) is expected to have materialised by 2025 and the remainder by around 2031. https://obr.uk/forecasts-in-depth/the-economy-forecast/brexit-analysis/

Key point: This 4 % lower GDP is a relative projection what the OBR estimates output would have been had the UK stayed in the EU. It is not a direct measurement of a weekly cost equal to a cash payment. It’s a modelling assumption about relative economic performance, not a tally of money the government pays out every week. https://obr.uk/forecasts-in-depth/the-economy-forecast/brexit-analysis/

How This Appears in the OBR’s Actual Forecasts

• In its most recent forecasts (e.g., the March and November 2025 Economic and Fiscal Outlooks), the OBR continues to assume lower trade intensity and weaker productivity growth because of Brexit. They embed that 15 % lower trade assumption into their baseline projection for GDP growth and the UK’s economic structure.

• The OBR’s latest forecasts show relatively modest growth for the UK economy, but do not separately itemise “Brexit costs” as an explicit weekly amount. Instead, Brexit effects are implicit in the slower growth trajectory. https://obr.uk/box/how-are-our-brexit-forecasting-assumptions-performing/

For example, the OBR’s macro forecasts for 2024–29 (in its Spring or Budget documents) show:

https://www.gov.uk/government/publications/spring-statement-2025-document/spring-statement-2025-html

Impact on Public Finances

The OBR also translates economic performance into public finances (tax receipts, spending, borrowing):

• Lower productivity and slower GDP growth mean less tax revenue and weaker balancing of public finances than would otherwise have occurred.

• For instance, downgrades to productivity assumptions (which are partly linked to Brexit’s longer-term effects) have been linked in commentary to a £20 billion or more deterioration in fiscal headroom by 2029–30. https://www.theguardian.com/business/2025/oct/28/rachel-reeves-uk-productivity-obr-tax-rises-budget

Again, that’s not a literal weekly loss, but a cumulative one relative to a counterfactual scenario where the UK achieved stronger productivity growth.

Why the “£850m per Week” Number Emerged

Analyses by independent economists have taken the OBR’s assumption of 4 % to 8% lower GDP over time, converted that into a total lost output (in cash terms) over years, and then divided that by the number of weeks to express it as a weekly figure. That’s how figures like £500m–£850m per week get cited in media it’s a calculated rate of lost output compared with a hypothetical non-Brexit baseline.

But:

• This is not a cash sum the government is paying.

• It’s not an OBR headline figure.

• It’s a modelling interpretation that depends heavily on assumptions about what the UK economy would have been like without Brexit. https://questions-statements.parliament.uk/written-questions/detail/2025-10-21/HL11209/

Summary of Official OBR Position

1. OBR assumes Brexit reduces long-run trade intensity with the EU by ~15 %.

2. That is modelled to reduce potential GDP by about 4 % over time relative to a remain scenario.

3. The OBR forecasts growth reflecting these assumptions, but does not state Brexit “costs” in weekly cash figures.

4. Estimates like £850m/week come from dividing estimated lost GDP output over time by the number of weeks an analytical construct, not an official statistic.

Weekly and Yearly Cost of Brexit to the UK the Calculation

Straight numerical conversion of the Office for Budget Responsibility’s (OBR) assumption that Brexit has reduced the UK’s GDP level by around 4% relative to a hypothetical “remain” scenario* into annual and weekly cost equivalents — using the best available recent UK GDP data. Importantly, this is a modelled relative loss, not a government expenditure figure.

Step 1 — Current UK GDP (Nominal)

The most recent official figure for the UK’s total GDP in cash terms is:

• About £3,038 billion (i.e. £3.038 trillion) in 2025 (ONS estimates). https://commonslibrary.parliament.uk/research-briefings/sn02783/

We will use this as the baseline “UK economy size in money terms.”

Step 2 — Apply the OBR’s 4% GDP Reduction Assumption

Assuming Brexit has made the UK 4% smaller (in GDP level terms) than it otherwise would have been:

• 4 % of £3,038 billion = £121.5 billion

• 8 % of £3,038 billion = £243.0 billion

This means, in the OBR’s framework, the UK’s economy is £121.5 billion smaller in a given year than it would have been without Brexit.

Step 3 — Convert to Annual Cost Equivalent

• Annual “Brexit GDP loss” = £121.5 billion per year @4% or £243.0 billion per year @8%

This is the conceptual annual difference in output compared with a remain baseline.

Step 4 — Convert to Weekly Equivalent

There are 52 weeks in a year, so:

• £121.5 billion ÷ 52 = £2.337 billion per week @4%

• £243.0 billion ÷ 52 = £4.673 billion per week @8%

So on this basis, the economy’s reduced output relative to the counterfactual equates to roughly:

• £2.34 billion per week @4%

• £4.67 billion per week @8%

Notes on this calculation

1. This is not “cash the Government pays out.”
It’s a relative measure of lower economic output compared with a hypothetical scenario where the UK remained in the EU. It’s not money flowing out of the Treasury each week; it’s lost economic activity.

2. The baseline figure (£3,038 bn) is nominal GDP.
Nominal GDP includes inflation and price level effects not just real output. This means that the £121.5 billion figure includes not only production changes but also price effects embedded in nominal GDP. https://commonslibrary.parliament.uk/research-briefings/sn02783/

3. The OBR’s 4% assumption is a long-term structural effect, not an annual forecast.
The 4% difference is meant to represent a permanent gap between a remain scenario and the actual projected UK economy over the medium to long term. It isn’t something the OBR literally reports as an annual “tax” or government cost. It’s a modelling assumption about economic performance over decades.

If you translate the OBR’s 4% lower GDP assumption into a cash equivalent, the result is closer to £2.3 billion per week in lost economic output relative to a remain counterfactual — far higher than the £850 million figure often quoted in media, because the underlying GDP base is larger and the 4% gap applies to the whole economy.

Opinion

Now if I was to LIE like the pro-Brexit people the figure on the bus should not be £850 million a week or even the £2.3 billion per week but by the factors they used it should be between £5.35 billion and £5.0 billion per week but that would be propaganda.

So when you look at the figures why would any sane person want to cripple their own country and steal the future from their children.

All I can think is it suited someones political and monetary gain, who gained from this after all? Individual politicians, but also countries outside the EU who felt threatened by its strength.  Certainly at the time there was a lot of money going to causes to fight for independence from within the EU and now its going to extremist political groups instead.