Heathrow have a version on the UK system,with slight differences from what we use up north within Nats.For example Light following a Heavy in the UK Mats part1 is 8 miles.At LL it's 7.Light following a lower medium would be 6miles.At LL it's 5.
I suppose they could argue that they are very busy,and it's as near to ICAO standard that they can lose a mile.If I put a light 5miles behind a lower medium,it could well cause problems.LL it's OK.
With helis such as the AS332 (Small) and say a Saab 340 behind(Light) the requirement is 4 miles.With the speed differential,especially if the heli want 80kts inside 4 miles,then we always go for the buffer.Aim is to get 4miles when the heli is at touchdown.We don't have the 1/2 mile below standard at all.