Here are the meeting spaces I've used for founders/braintrust meetings:
In Silicon Roundabout (Old St)
Campus London http://www.campuslondon.com/ - 4-5 Bonhill St - 5 mins from Old st Tube. The basement has a huge cafe where startups and techie meet. Plenty of plugs, coffee etc. They let out the place to Central Working, but it's free. And there's a cafe with pretty decent food. Otherwise people visit the sandwich places nearby and bring the food to talk there. (Benugo is quite good, by Exit 5 at Old St Tube). Obviously free wifi and all that.
In Piccadilly Area:
YumChaa (http://www.yumchaa.com/tresc/soho/13/) 45 Berwick St. About a 5-7 minute walk from Piccadilly Circus tube. Serves tea, sandwiches and has great wifi. Get there early, but there's seating downstairs.
Nordic Bakery - http://www.nordicbakery.com/ It's in Golden Square (3-5 minutes from Piccadilly Circus. It's quiet. The food is really only good if you want coffee and cakes.. but then again I wasn't there at lunch. Not sure if there wifi. Its main selling point is good coffee and its quiet. I had all my braintrust meetings there.
There are Central Workings everywhere and the have food (cafes built in), but they charge you for a meeting room. They aim to fit the gap of Cafe + Optimized Office Space.. but they are expensive (if you don't go to the one in Google Campus).