This one again. "No ATC speed restriction" means that ATC have no requirement for a particular speed to be flown to achieve/ maintain separation or to help sequencing, it does NOT absolve pilots from the rules of the air, therefore 250kts in appropriate airspace is still applicable. In and out of LHR you are surrounded by class A airspace and (I think) the SIDs specify 250 kts so the controller is a) at liberty to approve +250kts below FL100 and b), if he does, he is lifting an "ATC speed restriction" hence the use of said phrase. If you need greater than 250 to remain clean then it will probably be given, but always make your requirements known. A heavy aircraft is not entitled to go any faster than the rest just because it's bigger. Does it even need to in the descent?
As an aside, to those who fly into & out of the Belfast TMA which is not blessed with anything better than class E airspace (with a little bit of class D thrown in), I expect you to be doing 250kts or less by 22 DME from the BEL AT THE LATEST to comply with the 10,000 rule, even if told "no ATC speed restriction"- see line 1.