Have done the SVFR route Fairoaks - Ascot Racecourse - Burnham a couple of times. If I remember correct, you will be cleared by the folks at Heathrow "not above 1500" if they are on Westerlies and "not above 1000" (QNH
) if they are on Easterlies.
Have done that once and 900' on the QNH does feel low!
It is a spectacular ride though as the "heavies" trundle down the ILS about 1000' above.