<<Gatwick have special permission from the CAA to run 'reduced wake vortex separation minima' of 2.5nm (and I have heard of less than that).
But certainly in regard to arriving/departing aircraft, that reduces to 1.5nm. >>
Heathrow can also employ 2.5nm spacing. I know - I did it for many years. I have never seen less than 2.5nm and I do not believe that it is authorised anywhere in the UK. I don't know what you mean by the second sentence above..?? The final approach spacing would never intentionally be reduced to 1.5nm or the go-around rate would be enormous. Under normal wind conditions 5-6nm is needed to get departures away in the gaps. In very strong wind this can be reduced slightly..