A GP in London would be awesome.... for the 5,000 people that had corporate packages, flats overlooking the circuit, etc.
For the rest of the population of London it would be a f***ing disaster. Not just four days of transport chaos, but years of road-works for modifying various parts of central London, tube line 'upgrades', etc, etc.
For the drivers it would be both boring (a procession) and dangerous (teams pushing drivers to please sponsors).
Is it really worth p*ssing X million people off for two years just to give a few captains-of-industry anecdotes about drinking champers off a russian hooker while watching Schumacher roll to his 999th win?
All in all the London GP is one of the most ill-conceived, ill-advised ideas to come from politicans in a long time. Which means it'll almost definitely happen.

Get rid of F1 and get some real racing back on prime time tv.
Cheers,
Ali