FPOBN,
From what I understand, the spacing is based on a minimum measure crossing threshold...
At LHR, spacing and wake turbulence separation is applied to 4DME.
Curious how you get less than 2.5nm sep, with wake turbulence issues?
Well, of course, if there is a more constraining separation requirement, such as wake turbulence or diagonal separation due to an aircraft on approach to the parallel runway, then that applies. We only go below 3nm on pairs that are not wake constrained.
So for example, A320 followed by A320, or 737 followed by 767, etc.
As I said, we only go for 2.5nm when the conditions exist that when the 2.5nm begins to compress, i.e. when the first aircraft reaches 4DME and slows down, the Tower can apply visual separation.