After departing EGLL you climb to FL280 and maintain it using altitude hold and MACH hold (auto throttle mode) at M0.94. Once you reach waypoint UPGAS N54 21.0 W003 50.0 you can begin transonic acceleration/climb.
martinlest wrote: Actually, you are right of course for flights out of JFK. So let's use Heathrow departures as an example then, where I do need to level off at 28000' for a time...
.
UPGAS is waypoint 3 in the FS9 flightplan I gave you. It is in the Bristol Channel and means you can go supersonic without laying a boom over land.