Hi DFC
the examiners standards are just that; for when they are examining for a flight test!!! AS the UK AIP states for aerodromes in the london TMA, speed instructions are mandatory
As PPRUNE RADAR has shown there is no tolerance expressed in the UKAIP therefore if the pilot cannot fly the speed exactly, then ATC should be notified. In the LTMA speed control is used to maintain separation. When ATC are using 2 1/2 nm spacing it can go to a can of worms very quickly if the front aircraft is 10kts slower and the back one is 10kts faster