Funnily enough, I think there is a special note to tower controllers at LHR to cater for the fact that the A340 actually SLOWS DOWN as you describe. They have to beware of firing, say, a 757 off behind it because of catch up.
Just another point: If you are told 'no speed restriction' on departure you are released from the 250 kt limit, but you are NOT absolved from any level restrictions. If, by speeding up, your ANGLE of climb is reduced so you cannot meet 'level by' restrictions in the SID, then don't speed up!!