Hi Guinevere, I did the London Tri last year - sprint distance - it was the first time I'd taken part in any kind of sport since leaving school and it was absolutely brilliant experience.
The organizers of the London Tri are very helpful to first timers, they do training days and answered any questions I e-mailed to them.
I've just skimmed through this thread and you I think you said you were most concerned about the swim - I was too. For me starting to swim in wetsuit was the hardest thing as I only swim breast stroke. I think the best advice I can give is start training in a wetsuit by April at the latest. Also find a large outdoor pool, Tooting Lido or the Serpertine for me. If you swim front crawl you shouldn't have the same problems as me, but if you swim breast stroke I found a windsurfing shortie wetsuit better as it doesn't restrict elbow and knee flexing the way a full length one does.
For the race don't let yourself get exhusted by the swim, just go at an easy pace for you and think about making up time on the road. I was nearly last out of the water but had a huge smile on my face as went by the others on the bike and run.
I ended up doing far better than I expected to. I started out just wanting to finish but as I trained I aimed for 2 hours and then 1hr 45min and ended up finishing in 1hr 36min.
Go on enter you'll love it.