I agree that Didcot is not ideal, London would be best for me. I've suggested St Andrew's Crypt to m0ds before. It has no wifi or alcohol, which is probably why it was rejected, but it's relatively cheap and spooky and near Holborn Circus:
I agree that it would be good to broaden out from AGS and try to draw a less super-cool-but-sort-of-narrow crowd, which I think is your original goal.
I think you could also focus on making things more developer-y. Some kind of 48 hour adventure game-making challenge? If the event did take place at an interesting location, you could make 'Adventure X: The Adventure!' as a way of drawing together / promoting the videos and material generated during the 2 days.
I think it would also benefit from a stronger itinerary. As well as a few more presentations, I think the raffle could be replaced by an adventure game pub-quiz, or a treasure hunt!
It would be great to have a documentary team with a mandate to shoot a full behind-the-scenes video, in addition to shooting the talks. A second camera and some mics would help up the production values and make a great advert for next Adventure XIII!
I'll be there! This year I plan to drink more beer and pester Ali less.
I demand to be pestered more!