n the UK it is generally believed by ATC that a clearance for an approach permits the aircraft to descend immediately to the charted altitude at the IAF.
...
If true, where is it written?
If it were written somewhere I could find, I wouldn't have said "it is generally believed".
However, this is from the CAA's
CAP413 supplement
"‘Cleared ILS approach’ may introduce an element of uncertainty as to when descent will be initiated because the pilot may descend to the final approach point altitude (platform height) at any time after receiving this clearance.’
There are also many threads on PPrune about the non-standard UK phraseology in which this aspect is discussed.
I believe that both interpretations are reasonable ones (without further clarification as in the AIM).
A clearance for the approach is an instruction to follow the vertical profile of the approach without further ATC instruction. Descent to subsequent levels is implicit. Why should that implicit vertical profile not start at the IAF with the charted altitude there?
Another example: I'm being vectored for an ILS at 3000 ft, a level above the charted FAP altitude of 2400 ft. The controller says "closing the localiser from the left, cleared ILS approach runway xx". I intercept the localiser and the glideslope comes in 2 miles before the FAP. May I descend? By your reasoning, I'm not on a
published segment of the approach yet, so don't I have to wait until I reach the FAP before descending?