Another odd thing is that these recalculations are done every year, so the Treasury would know that it was going to happen. I think it has been done this way since 1995, at the insistence of various governments, including the UK. Ironically, of course, it has happened because of the British economy doing better than previously anticipated.
I realized that the Daily Mail published an article on 29 May, saying that the economy was being boosted by various extra activities, including prostitution! They calculate this as an extra £10 billion. Does the Treasury not read the Daily Mail?