True, but we can use a reasonable mathematical model to come up with a good approximation. We have 40odd years of data to work with.
The assumption is that membership in the EU is the dominating factor in the change of growth levels. You'll need to prove that is the case first. Good luck with that.
If you try to do a serious analysis, don't bother. It was already done, by multiple experts - actual experts in the field, not politicians - and they only disagree on how bad the whole thing will be for the UK. They're unanimous on it would be bad for the UK economically and otherwise.
In other words, it was all already done, and it was dismissed out of hand by the Brexitards. "This country has had enough of experts" is typically not a good way to start making your plans. Since it's clear there was no analysis and no plan made for the whole thing, the UK needs a miracle to avoid disaster.
McHrozni