alphaalpha,
Do the London TMA controllers working the area close to EGLL want to hear from GA traffic just under the TMA base? I assume not, on workload grounds. (HD or 120.4 any comments?)
Not on any of the Heathrow Approach or London Control frequencies.
But if you contact any of the other radar units in the area (Special, Farnborough, Benson, Northolt etc) and we see you on one of their squawks we can assume they have verified your height readout and are looking after you.
WF.