If you want to play Ultimate in London there are two things you should do straight away:
The LondonLeague mailing list is primarily used for Summer and Winter League announcements. You'll also hear about other local events, and teams sometimes post there looking for players. Anyone is welcome to use the list if they need to get in touch with the London Ultimate player base at large. Don't worry, there's not much traffic, so your in-box won't suddenly start overflowing.
This is a web-based discussion forum, with chat about Summer and Winter League, and lots of other good stuff. League organisers post updates there too, which don't always appear on the league website. It also has forums designed to connect players with teams and vice versa. These "players available/players wanted" forums get particularly active before the start of Summer and Winter League (around April and October respectively).
Visit the forum at www.londonultimate.com/forum
BritDisc is a national mailing list for announcements and general chat about Ultimate across the UK. If you are interested in playing events like the Tour, or any other tournaments outside of London, you should definitely sign up to BritDisc.