I have helped organize several similar events in the past, I can recommend looking at Moat House/Holiday Inn type places.
These hotels will do special room deals for people attending events, have function rooms of varying sizes and the appropriate infrastructure to deal with an event.
I would also suggest pre-booking being a requirement for people attending to give you an idea of numbers.
Hotels such as this will almost always have good road, rail and bus links too.
I'm sorry but I know from experience that having to fight your way across London on foot with baggage really takes the fun out of the event, knowing as soon as it's over you have to do the same thing all over again to return to your train/plane/park & ride.
Where as I'm sure London is convenient for people that live there and are used to using it's transport network (and I lived there myself), it's twice the trouble (and probably cost) for anyone traveling in.
Just my $0.02