I think it stems from an incident years ago when a pilot, having been cleared for the approach, descended to MDA as soon as he had captured the localiser. Whilst I have heard it argued that this is a legitimate practice, it does rather cause problems if the controller has something going on below the approaching aircraft (like Heathrow zone heli routes).
It's not legitimate to descend to the MDA, but when "cleared for the approach" it
is permitted to descend to the charted intercept altitude (where you'd be for a procedural approach). So if you get the clearance for the approach at 12 miles and 3000 ft with a FAP at 5 miles and 1500 ft, you can descend immediately to 1500 ft, which may conflict with traffic below.
Every other country manages this by a simple phraseology like "cleared for the ILS approach, maintain 3000 ft to the glideslope" where necessary. The UK has to be non-standard and ends up with more words and a phraseology that never includes an explicit clearance for the approach. Hmm.