One of the problems of ATC can be a lack of consistency, in that one person's plan is not the same as the next, and our aim of a seamless exchange sometimes falls over. This seems to be one of those cases. You might say of course that the earlier speed control was so effective that BHX approach were then able to allow you free speed as the distance between you and the flying brick was enough.
The question of Mach restrictions in the cruise is one of those which RVSM might help to resolve, as there are twice as many levels available.
Again, it depends on the controller. I will usually offer you a choice between speed control and a lower level, and most of us don't try to speed 747s behind 737s etc. I consider 747s to be impossible to speed behind anything other than another 747, A340s are OK in front of 767s or other Airbuses, A320s etc are OK with 757s but not 737s, and 737s and CRJs/EMBs are OK once the RJs get levelled off and build speed.
Then of course you have the rpoblem you have mentioned of different operators of the same type having a differen speed schedule.
Never simple, is it??
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"Take-off is optional, Landing is mandatory"