mm_flynn,
The challenge for the Heathrow people is that the London Zone, being Class A, needs you to be separated by 3nm and/or 1,000ft from anything else in the Zone.
So if say someone beat you to it and was coming in the opposite direction via Ascot and Burnham, there isn't enough room available to apply the separation standards and accomodate you southbound at the same time.
EGLF would have to co-ordinate with EGLL before clearing you in to the London Zone, they are not allowed to do so off their own back, so whatever was in the way to stop your intital transit must have moved out of the way by the time you were cleared in by EGLF.
WF.