luke77,
I believe that aircraft are allowed 10 kts either side of the instructed ATC speed? If one`s situational awareness from TCAS ETC is clear , then a few knots from the exact instructed speed for SOPs should be fine without asking on the busy frequency.
The +/-10kts allowance is incorrect.
Assigned speeds should be flown "as accurately as possible" says the UK AIP. Two aircraft on final, one deciding to fly 10kts slow and the one behind 10kts fast could be embarrasing for all concerned
The AIP info on speed control into Heathrow is
here.
WF.