Jesus christ, this is another one straight out of the "How-to lie with Statistics" book, but not from ONS, just how it's been interpreted.
If you don't like the result, change how you calculate the result.
Even better if it's a complicated calculation, that nobody understands, other than the one doing it. I do this all the time at work if I know the requirement for the calculation is practically pointless anyway, and I'm not getting the result I want to show. Obviously, this is a bit more important.
I'm not saying ONS are lying, they're not, They've just changed a calculation or the variables, and the result is probably correct based on that calculation.
The key thing is why have they changed the calculation or variables, have they done that on their own initiative, or have they been asked to by who funds them (the government).
However, no matter how you change the calculation or variables, it's not going to polish the brexit turd.
It's also a subjective calculation, hard to compare various countries, and if they used the metrics we are then their GDP may get a boost also. I expect their governments/ state funders have a bit more honest intentions.
We had the fastest-growing GDP in the G7 before the brexit vote, now we've got the least, and we have the worst forecast too.