I think the point made above about the 4nm cut off is critical. The wake separation applied in the UK only applies until the first aircraft is at 4nm. The first aircraft then reduces to threshold speed so some distance compression will occur; however as the second aircraft will also reduce speed from 4nm to the threshold the time separation between the two flights will remain constant or at least very similar.
AFAIK this arguement has always been accepted by the CAA.
If you applied vortex spacing to the threshold at Heathrow from today the movement rate would fall off a cliff.
Wake vortex reports are taken very seriously by both NATS and the CAA and I have never known anyone attempt to cover any up.