It is your money, use it as you please and when you please. The assessment is complete and the decision has been made, no one is actively looking at your bank statement anymore.
Except the IO who will interview you once you land, you may be asked for a proof of funds for your trip at that point. And if by then you are no longer in possession of enough funds for your trip, you may be denied entry.
Obviously it is normal and actually expected that by the time you reach the UK, you would have spent some of your money on tickets and other arrangements for your trip, so no one will hold you to the exact same amount you presented on the application stage.
Credit:stackoverflow.com‘