You'll find plenty of great stuff on the web to help you learn Ultimate. See our links page for some of the most useful resources. A few other sites that new players will find particularly useful include:
What is Ultimate - rules insights
Simple explanations of some of the trickier-to-understand parts of the rules.
Video papers - Ultimate skills
A selection of videos illustrating various aspects of the game - strategy, skills and fitness training. Most of it deals with fairly advanced-level concepts, but the section on throwing will be of immediate benefit to newcomers. Authoritative stuff from some of the world's best players and teams.
In the UK, try Lookfly for books and DVDs about Ultimate.
One of the initial hurdles for new players is simply learning to throw well. There's no substitute for putting in the hours with a disc. The majority of your time during games and practices will be spent without a disc in your hands, so relying on team practices to hone your throwing skills will make for slower progress than necessary.
One solution is to find another player who is happy to go out and spend an hour with you just throwing and catching. Maybe agree to show up early for team practices, or find other times to go to the park and have fun with a disc. Even the best players are continually working on their throwing.
Occasional "skills days" take place in London, run by players from some of the best teams in the city. Watch the forum and mailing list for details. If you've started your own team, you could also ask an experienced player or two to come along to your practices from time to time. Alternatively, try to get a couple of experienced players to join your summer league team.
Track down a local team and arrange a friendly game against them. It's often worth approaching teams that are slightly more experienced than yours to see if they'll give you a game - after they've comfortably beaten you (or not) you can "mix it up" by combining your players and splitting into two evenly matched teams.
One of the fastest ways to improve your Ultimate is to play competitively. Summer League and Winter League are good options for Londoners, and avoid the commitment and logistical issues you must deal with to get a team to a regular tournament. On the other hand, weekend tournaments are like taking a crash course in Ultimate and are incredible fun in their own right. Not only will you gain a lot of competition experience in just two days, but you will learn a lot by simply watching and trying to analyse other teams' games.