London just isn’t a nice place to drive a car, but I manged to come up with something pretty sweet, though not feasible in practice: http://www.gmap-pedometer.com/?r=5537062 (counterclockwise)
It goes past famous landmarks like the Tower Bridge, the Tower of London, the Gherkin, City Hall and St. Paul’s Cathedral. The fast chicane before Blackfriars Bridge seems wicked. Also funny that it happens to unintentionally look a lot like the old Silverstone.
More practical, but less interesting is this one: http://www.gmap-pedometer.com/?r=5537100
It runs through Westminster and also has a nice vista of Battersea Power Station.