OK.. but remember I'm retired and don't want to tread on any toes..
This is precisely why we have the strange phraseology in the UK, to prevent anything untoward occurring. If, as a controller anywhere in the world, I cleared someone for an ILS (or other type of approach) I would expect the pilot to do what he deemed necessary to complete the approach in accordance with HIS procedures. I.e in the scenario referred to I would expect ATC to ensure separation in anticipation of the pilot descending to 2500ft. (Let's not get involved too much in the range of ILS installations).
At Heathrow, for the reason normal_nigel notes (among others), you won't be "cleared for the ILS" so the situation should not arise.