There's a difference between 2.5nm spacing and 2.5nm separation though.
2.5nm separation can be done at any time, if I remember correctly, between two a/c (non-vortex implications) within twenty miles from TD, if they're both either on base or final. Also you have to be using either the H23 or the H10 radars. There is no weather criteria for 2.5nm separation.
2.5nm spacing is offered by the tower at their discretion. I don't believe in the approach part 2 there is any specific criteria for it other than if the twr say its ok and you are fulfilling the requirements of 2.5nm separation. There was a whole list not long ago but its been done away with. There may still be some weather requirements in the twr part 2? We wouldn't ask for it though unless there was a reasonable headwind component on final, reasonable vis, and a dry runway.
2.5nm separation is a useful thing to have even if the tower have asked for standard 3nm spacing on final. One time it is extremely useful is when a/c are 'pulling apart' on final i.e. there is a much stronger upper headwind than lower on the approach. Effectively as a/c descend on the ILS their groundspeed increases pulling them away from the following a/c. 2.5nm separation means we can position a/c 2.5nm apart on final and then let them 'open up' to 3nm before we transfer them to the twr, thus not losing separation, giving the tower the final spacing they require, and keeping the landing rate up and the delays down.