Agree strongly, just because you have been ATC cleared does not mean that the rules of the air are suspended. It is up to the PIC to keep to these rules even if under Radar Vectors (maybe the ATCO has a responsibility if under vectors - ATCO2?). It'll be your arse and not the ATCO's that gets kicked if the CAA catch you.
I'm Biggin based and have often heard light singles asking Thames for clearance into the City Zone to do sightseeing or to transit(been talking about this and other things with ATCO2 over in the ATC forum). Unless I'm wrong, the whole area is congested, and as as has been said before, the alight-clear rule cannot be met by a light single over Greenwich at 2,400ft.