If you use the search link at the top of the page and search the topic of "London" for the last few years you will find that the subject gets discussed from time to time.
The short answer to your question is that the house edge is quite substantial. I've only played in a couple of London casinos and both were too small to wong. They do allow back betting, though, but you have to find someone who knows how to play to use that strategy (not easy to find in a London casino.)
There used to be a 24 hour waiting period between when you "joined" and when you could play, but you can play right away now. I don't know if you still have to "join", but I would guess that you do, and you'll have to show identification to do so since that is common practice in European casinos.