Hi HD, the problem is that 3 extra calls have to be made:
1. A/C "established on the loc"
2. ATC "Cleared for the ILS"
3. A/C readback "Cleared for the ILS"
A simple "cleared for the ILS, leave XXX' on the glide" make those three calls superfluous. I sure hate it when I'm told to report the loc when being vectored close to G/S intercept, only to find myself in a position where I can't get through on a busy freq. to get a clearance allowing me to follow the G/S. I can assure you that intercepting a G/S from above in a heavy A320 with still (or some tail)wind is not easy without busting FDM-limits.
Also I remember having read plenty of topics about this issue on pprune. I remember that I looked up and found ICOA (not UK!)documentation confirming that I can descent to published G/S intercept alt when cleared for the ILS on a intercept heading. I'm sad that I can't find it back at this time, but I'm sure someone will come up with an official ICAO document.
Best regards,
Sabenaboy