The ONS, like Eurostat is a genuinely useful organization full of people who care about the accuracy of numbers. When the original data was released, they were clear that there were likely to be revisions. As it turns out the revisions are bigger than expected but the justification that during the Corona crisis, businesses didn't prioritize the form filling required by the ONS, is reasonable. There is also a change in methodology for many of the global stats agency in how GDP is calculated. The ONS is actually ahead of the game in getting that implemented. It's also had a positive effect, one that is likely to be replicated in other countries as they adopt the changes.