This is just another manifestation of the police state that we now live in. There is no legislation about this (amount), but the police have just decided that no one has a legitimate reason for carting about more than £5k and will confiscate it until you can prove its legitimacy.
If you have a receipt from bank/bureau bring this. If not, at least bring some documents detailing the tournaments.
If you win in Monte, remember to bring along the photo of you with trophy, suitcase full of dosh and pokerstars model/host/girl person.

ps I'm going to win anyway so you probably won't have this problem.