If you're travelling to London from Europe by train you will likely/ certainly (?) arrive at London St. Pancras International. I met a friend there last month and we went to a great boardgame cafe called Rogues Quarter. We got the tube to Chalk Farm and it was a 10-minute walk from there. They had big tables, a good selection of games and nice, well-priced food.
Others I've visited in London include Draughts in central/ North London and D20 which is a bit further afield in Watford, but still worth a visit if you're in the area.
Hope you enjoy your visit!