Could you explain why you have taken that estimate at face value?
Your link says the modelling was carried out by an economist at the Centre for European Reform (CER).
ABOUT THE CER
The Centre for European Reform is a think-tank devoted to making the European Union work better and strengthening its role in the world.
https://www.cer.eu/about
Looking a little further on the think tank's website, I see that the economist is having to defend his estimate;
Critics say my estimate – that the British economy is around 5 per cent smaller due to Brexit – is implausibly large...
In a debate I participated in last month [April 2023] panellists were asked to say how much Brexit had curbed Britain’s economic output. Graham Gudgin of Policy Exchange and Cambridge University’s Centre for Business Research said he thought the costs had been negligible. Julian Jessop of the Institute of Economic Affairs said GDP was around 1 per cent smaller than it would have been, adding that the short-term impact was always going be negative, before the benefits of bespoke regulation and reduced trade barriers with the rest of the world kicked in. Jonathan Portes of UK in a Changing Europe and King’s College London said the costs amounted to around 2.5 per cent of lost GDP.
All their estimates were lower than mine – my model finds that Brexit had reduced the size of the economy by 5.5 per cent in the second quarter of 2022, and all three panellists expressed doubts about my estimate...
We've recently had revised economic data for the UK - so at the very least the economist from the CER needs to update his modelling:
September 2023
The UK actually fared much better after COVID than first thought.
The ONS unveiled a series of revisions for past GDP growth - affecting both 2020 and 2021.
Put together, it means that at the end of 2021 - rather than being 1.2% smaller than it was going into the pandemic as previously reported - the UK economy was actually 0.6% bigger.
https://news.sky.com/story/the-uk-actually-fared-much-better-after-covid-than-first-thought-heres-why-it-matters-12952220
UK GDP: Economy grew faster than Germany and France after Covid.
https://www.thetimes.co.uk/article/uk-gdp-economy-growth-business-ons-figures-2023-v952xvkg7