We hired a working barn for ours and did the whole thing up ourselves, had a pig roast and it was an absolute blast. Filled it up with wine from virgin wines so worked out really reasonable.
We also looked at hiring a boutique bowling alley in central London – which looked great and a whole lot cheaper than a usual venue.
Let me know if you want more details, I can also put you in touch with one of the guys at virgin for wine buying