What would you propose as a solution? The critical factor is safety. What's the easiest way of clearing someone for an ILS, using phraseology which he will hear in 60 different countries, which will guarantee separation from traffic underneath? "Cleared for the ILS" has been proved to be dangerous for us..
If you don't trust crews to do what PANS-OPS tells them they should, how about "Cleared for the ILS, maintain xx00 until fully established on the glideslope"?
I don't see what's so special about the UK that makes it a particular issue here. The critical factor is indeed safety, but the positives have to be balanced against the negatives of both misunderstandings through the phraseology being non-standard and the destabilishing effect that Capt Peacock describes. Safety is not simply about keeping aeroplanes apart -- it's about crew workload too.
I take your point that different phraseology is used elsewhere without the system falling apart, but the objective should be to create an optimal system, not just a system that is no worse than someone else's.