The 2.5nm approach spacing at Heathrow (and elsewhere?) has been in use for many years. I retired nearly 6 years ago and it was in force long before that so it has probably been in force for 10 years? It may only be used under certain conditions and between particular types of aircraft and only on final approach. Not every flight is subject to 2.5nm spacing. The simple reason for it is to achieve greater runway utilisation.