IRISHWINGS I think that you will find that those good people at EGBB will only be descending you from 2500' to 2000' IF you haven't yet called established on the Loc.
It is to ensure that whenever possible, you continue to "attack" the g/s from below if (say) your closing heading means that you might not be established until about 6 or 7 miles.
The further descent clearance (to 2000') can only be given within (about) 9 miles whilst descent to 2500' can be given from about 14 miles.