London Airport SIDs
I've noticed that the SIDs out of most of the London airports restrict departing aircraft to a relatively low altitude and a low airspeed for quite some time/distance. For example, all of the EGLL SIDs top out at 5000 ft or 6000 ft despite the fact that they cover up to 60 or 70 miles of lateral distance. I know there must be airspace limits to allow the traffic from all these airports to mix safely, but it seems a bit excessive to keep an airliner departing EGLL or EGKK to still be flying at 6000 ft/250 KIAS as they pass over the DVR VOR. Are these SIDs usually flown in their entirety, or is it common for ATC to approve an early release to enroute altitudes and speeds?