Exactly what I was going to say.excel centre in docklands
ExCel is probably the most obvious place in London. It's fricking enormous, you could fit 5 fields, staging area and the trade show in about half of it. There's no problem getting there either, it's got 2 stations on the DLR, road links are good and it's right next to London City Airport for those travelling from overseas. There's also plenty of parking there, and 2 or 3 large hotels so accomodation is no problem either. Other hotels in Canary Wharf and Greenwich are only a few minutes away by tube and DLR too.
The other massive bonus is that it's indoors so weather is not an issue.
Also, and most importantly, it's a high profile London venue and with a little publicity could really draw in the crowds. If Millennium charged something like £1 for spectators I reckon they'd be able to cover costs.
On the down side it would be hugely expensive to hire out for the 6 days or so that setting up, playing and tearing down would require.