I lived in outer-ish parts of London (Putney, Slough and quite a few other random places) for many years, and they are certainly not massively cheaper than central London.
The FACT is that somewhere like Cambridge has better, cheaper and easier to use road, rail and bus links, has cheaper accomodation & facilities and most importantly CHEAPER BEER for us northerners who are used to Lancashire Brown Ale at £1.50 a pint!
If this were a large sponsored corporate event and everyone attending was on 50k a year then London would be fine, but alienating large ammounts of the community and 85% of the UK seems insane.
Say what you like, commuting into London from anywhere else in the UK is a nightmare, especially if you're unfamiliar with London's public transport system and many people will find that intimidation a reason not to attend.
I drive to Cambridge quite a bit (Huntingdon to be exact but I visit Cambridge) and it's a piece of piss to get to.