Foghorn
No problems - give Thames Radar (132.7) a call and 99% of the time they'll let you transit their zone VFR from Thames Barrier to Lea Valley (providing, of course, that they have the time to deal with you). Don't go further West than the transit instructions though, prepare to orbit if anything is on the ILS to LCY or remain clear and route down to Dartford Bridge if requested. The folks at Thames Radar are great - as well as all the professional traffic they deal with they do an excellent job of getting poor PPL's like me vectored onto the localiser at Biggin as well! Well worth talking to them as on a clear day the views from 2300ft qnh are fantastic and with proper clearance you'll be breaking no rules.
I doubt we'd get the OK to take a C152 over Westminster but I agree with Mark 1 that Sunday was a proper gesture.