London is a damn big place, so organising an all UK gathering there will get tricky...
Damn I'd be willing to go, even though I'm from "way up north"...
To be honest, if you get the Megabus service, or national express coaches it wouldn't cost more than £15 if booked about a month in advance, they all stop at Victoria station, and you can always get the night coach, which is a little more expensive, but it means no need for accomodation if it's a 1 day event...
LAN party sounds the best idea, I mean bowling wouldn't be bad, but it's expensive and doesn't last too long. If there is a place in London, like this little place near me called CGZ, which it should have because it is the bloody capital, you can get all-nighter LAN sessions for about £20 a person, it works out at about 10-14 hours gaming. That plus no sleep, loads of caffiene, and raman, people get really silly, though some also get really dickish, so we'd need to try and keep them in line, it spoils a big session when a couple just feck off and get bitchy...
Regional organisers, once again, sounds like a good idea, it means you could end up with teams and all, but most likely very uneven, and spreads out the organising, but if you just have one head doing all the work, it seems, and if the numbers got really big, would be a bit unfair...
If things get a little more sorted I'll definitely take part, but at the moment this seems like a dream rather than something that's going to definitely happen...