UK AIP London Heathrow Aerodrome section EGLL 1-18
"Speed Control: Pilots should typically expect the following speed restrictions to be enforced; 220kts from the holding facility during the initial approach phase; 180kts on base leg/closing headings to final approach; between 180kts and 160kts when established on final approach and thereafter 160kts to 4 DME. These speeds are applied for ATC separation purposes and are
mandatory. In the event of a new (non speed related) ATC instruction being issued (eg an instruction to descend on ILS) pilots shall continue to maintain the
previously allocated speed. All speeds should be flown as accurately as possible. Aircraft unable to conform to these speeds must inform ATC and state what speeds can be used. In the interests of accurate spacing, pilots are requested to comply with speed adjustments as promptly as is feasible within their own operational constraints. Pilots should advise ATC if circumstances necessitate a change of speed for aircraft performance reasons."
In high traffic density environment, where a high landing rate is necessary, such constraints are essential. This is particularly essential when we are applying 2.5mile separation, there is little or no room for error, hence the above. The caveat being, if you can't comply, let us know and we will compensate.
Hope that helps
