The Victoria (Vic) Casino offers pot limit games from GBP50 buy-in upwards. The Gutshot cardroom offer mainly small pot-limit games with buy-ins of GBP25 (fixed) GBP50 minimum and GBP100 minimum with occasional higher games. Both have websites you can google.

I think some more places have opened recxently but have not lived in London for a year so dunno. remember to take a passport with you when you go to either in order to get your membership.

If you want to play really high stakes, the Vic will be your best bet.