Bookworm,
The Luton QNH is available via their ATIS. The London QNH is broadcast on the Heathrow ATIS frequency
and on all of the London area VOR idents, if your NAV box has a voice facility.
We use our second VHF box to keep up to speed on the relevant local QNH via ATIS. We try to have the relevant QNH set before we call the ATC unit to which it belongs, so our altitude is immediately comparable to that of other aircraft they are working. We don't use RPS unless nothing more accurate is available, such as more than 25 nms away from an ATC unit and heading for the hills.
Had you "bust" airspace it is your good self, as the pilot in command, who would be found primarily at fault.
However, a second call to Brize, requesting the London QNH would surely have solved your dilemma?
