The job.
The ATC controllers job at LHR (or anywhere else) is to fit in as many aircraft as possible, and not compromise safety (read aircraft separation).
A very basic fact.
The aeroplane pilots job is to follow the ATC controllers instructions, so said aircraft separation is not compromised.
Another basic fact.
To expect the controller to 'know' just which aircraft is light/heavy (and therefore the 'ideal' speed for same) is complete nonsense.
A suggestion for a few, who seem to think that 'they' are the only aircraft in the sky, and all ATC procedures should be structured for 'them'.... do as the ATC tells you to, and stop complaining.
IE; leave the guy alone to do the job for which he is properly trained and certificated.