OK, since you're based in London, I did some searching for some of my club mates in that area and found the USS Britannia.
The Britannia is a chapter of Starfleet, The International Star Trek Fan Association, a world-wide organization which has been in existence since 1974.
According to their chapter web site, the Britannia is into role-playing big time and would probably appreciate another player / narrator. I'm not sure if they're doing table-top gaming with Decipher's rules set rather than some other kind of role-playing (like a sim or free-form persona role-playing). Still, they'd be worth checking out for players. If you find you like the group enough, maybe you'll join Starfleet too and partake of the other social activities that Starfleet chapters have to offer.
You can e-mail the Britannia's chapter chairperson at co@uss-britannia.org You can also e-mail the Regional Coordinator for the British Isles at rc@region20.org
Hope this information proves useful.
Ezri's Toy
Starfleet member since 1987.
"The American Eagle needs both a right wing and a left wing in order to fly."
-paraphrase of Bill Moyers