The Fox is one of the best places to play poker in London. It’s not really a casino as the majority of the floor is made up of poker tables, but hidden in the back by the bar is an electronic roulette machine and a blackjack table.
There are multiple daily tournaments (usually one starting in the afternoon and another early evening) and cash games going on all day. It’s apparently open 24 hours but I’ve never been there late enough to be able to say if there are still people to play against at 5am.
The fact they charge a membership fee to join is a bit annoying (£10) so if you’re only planning on going once I’d head to the empire which is a couple of minutes walk away in leicester square.