The two best places to start are by:
1= contacting warren "tiger jedi", who runs his ashford site and is running regular competitions which loads of teams will obviously be attending
2= turn up at one of the Dartford7's and again speak to the teams
3= Go to the euro5man this weekend at ashford, rumour has it there may be one or two teams there?
A word of advice tho, you will probably need your own gear to start out, most teams would probably lend you something first time out or if your gear breaks but don't expect free gear the first time out, sponsorship in paintball isn't that generous !