yeah, alright then:

you are arguing from ignorance, ask for evidence, then reject said evidence in the first paragraph instead of reading the entire thing because of a boilerplate disclaimer (which you of course do not understand to be boilerplate).

you read the executive summary, even though you asked for the methodology, which is explained in the studies linked under the sources of the article.

you need to click through to the actual study to see the methodology.

the link i provided is just a summary of multiple studies.

the studies lack this disclaimer, which was added by factually, probably for legal reasons, not because the data is faulty.

since you’re apparently too lazy to even click the links already pointing to the exact information you asked for, here’s the abstract of the NBER/Stanford paper (most relevant part at the end highlighted):

This paper examines the impact of the UK’s decision to leave the European Union (Brexit) in 2016. Using almost a decade of data since the referendum, we combine simulations based on macro data with estimates derived from micro data collected through our Decision Maker Panel survey. These estimates suggest that by 2025, Brexit had reduced UK GDP by 6% to 8%, with the impact accumulating gradually over time. We estimate that investment was reduced by between 12% and 18%, employment by 3% to 4% and productivity by 3% to 4%. These large negative impacts reflect a combination of elevated uncertainty, reduced demand, diverted management time, and increased misallocation of resources from a protracted Brexit process. Comparing these with contemporary forecasts – providing a rare macro example to complement the burgeoning micro-literature of social science predictions – shows that these forecasts were accurate over a 5-year horizon, but they underestimated the impact over a decade

from the CEPR/VoxEU article (already in plain language and easy to read):

So, taking all this together, what’s the bottom line? First, the public is right. Brexit has damaged the UK economy. But, inevitably, the mechanisms and hence the impacts have been considerably more complex than economists could incorporate in macroeconomic or trade models, with their inevitably simplifying assumptions. To simplify hugely, however, it would be reasonable to say that the impact on trade overall has been broadly consistent with predictions so far, that on immigration much less negative (and perhaps even positive) and on investment somewhat worse.

so, yes, brexit has been bad for the UK economy. definitely, without question.

what IS still in question is how bad exactly it was.

THAT’S were the uncertainty is.

whether or not it was detrimental has been answered with abundant certainty: it was bad.