I don't see how you can 'glide clear' in a C152 from anything less than about FL80 on that route. And in case there's any doubt, that's entirely your problem, Dimensional, not ATCO Two's.
I even worry about the 1500 ft rule in a twin. Under VFR, requiring 1500 ft above the highest object within 600 m means that transit of London at less than about 1900 ft is somewhat dubious.
ATCO Two, how easy is it for Thames to grant SVFR in perfect weather for the purpose of avoiding that problem? Seems daft to require ATC separation to avoid the low flying rules but...