Instead of trying to find an established team to join, why not start your own? There's a definite satisfaction and camaraderie that you only get by competing with a team that you helped form.
Obviously, start by asking your friends. It's easy to convince people to try a new sport when they know everyone else on the team is a beginner too. Maybe you can put a team together with work-mates or school/college friends. Find additional team members by advertising on the "Players Wanted" section of the forum.
You can start learning Ultimate with as few as eight people - just play four-a-side on a small pitch. However, to compete in most events you'll need a practical minimum of twelve or fourteen reasonably committed players. You'll also need to bear in mind the requirements of any competition you want to enter. For instance, London Summer League is played mixed, so you will need both men and women on the team.
There are lots of resources on the web you can use to help you get started. Occasional coaching sessions are organised in London for new players. See our learning the game page for suggestions.
You could simply practice amongst yourselves. But half the fun of Ultimate is meeting other teams, which mainly happens at tournaments and other competitions. It might seem daunting, but select the right events to start out with and you'll be fine. Ultimate is an incredibly friendly sport, and if you tell teams you meet that you're new to the game, they will do their best to help you out. Don't be afraid to ask questions - all of us were beginners at some point.
London Summer League is probably the least intimidating introduction to the sport for new teams, although Winter League is huge fun too - as long as you're prepared for the weather and getting up early on a sunday! Also consider registering with Ladder League and setting up some games. If you've formed a team at work, there are occasional tournaments in London specifically for corporate teams - keep an eye on the forum. If you're at school, check out the junior competitive scene (see the UKUA website).
As you gain confidence you might consider entering some weekend tournaments. Many new players became hooked on the sport instantly after attending a tournament... you only need to try one yourself to understand why.
There is a kind of "critical mass" effect with Ultimate teams - below a certain number of players it becomes difficult to keep the team going. Bear in mind how many players you need to practice effectively. If you want to be able to practice with "sevens", you need fourteen players minimum showing up... which means you'll definitely need more than fourteen "on the books".
You can have fun at practice even if you only have six (learn to play mini or hotbox). But if you only get five players out to a practice, it's seriously time to worry. Be prepared to scout around for new players as soon as attendance starts falling. Ask your team-mates if they have friends who might want to try the game.
Luckily, there are so many new players in London looking for teams, you should have no problem filling any gaps that arise. Make good use of the forum.
Running a team can be hard work, especially in the early stages. Start a mailing list (at YahooGroups for instance) so that you can all communicate effectively. Design a team logo, get a "strip" made (it need only be a couple of T-shirts per player), maybe even set up a web site. Be organised and tenacious about collecting money for tournament fees and other expenses, otherwise you can find yourself seriously out of pocket. Keep encouraging your players to come out to practices and games, and make sure you have some laughs together off the pitch too.