There are quite a few issues causing confusion here and if they are removed then I think the solution is easy.
1. Bookworm spoke about the UK practice of ATC not using the phrase "cleared ILS". This highlights a misunderstanding on the Pilot side.
Remember that when Approach radar are vectoring the aircraft towards the ILS, the aircraft is in the initial approach phase - the vectors are replacing the procedure turn, base turn or other procedure designed for the initial phase and once established on the LOC it is in the intermediate phase and when it intercepts the glide path (at whatever altitude within the glidepath covereage) it is in the final phase of the approach.
Therefore that situation is not the same as the one described in the original post becasue in effect when put on the 30 degree leg and told "cleared ILS" the aircraft is already on the initial approach leg (in effect inside the IAF) and by definition can (and should be safe to in terms of terrain) descend to the intermediate altitude.
That is not the situation here.
2. A clearance has a start point and an end. In this case the clearance "cleared ILS Z" while enroute to the IAF ony has effect when the IAF is reached. This seems to not only be a problem in the arrival phase but has also caused problems enroute where for example aircraft within domestic airspace have received an oceanic clearance at say FL380 and have incorrectly climbed while in domestic airspace to FL380 without ATC clearance. So every clearance has a start and a finish and one can not pre-empt the start.
In this case, the aircraft was cleared to proceed from the IAF through the procedure all the way to the missed approach holding fix. Anything before the IAF was a separate issue.
3. Everyone is having a bit of tunnel vision with regard to MSA and terrain issues. What about other traffic and airspace issues. In terrain terms is may be safe to descend to 2500ft but will that cause the aircraft to leave controlled airspace, enter a restricted area or conflict with other traffic?
I would be interested to know what the inbound track to the IAF of the original poster was.
The only safe option in this case is to request descent when appropriate because "cleared ILS" being issues to an aircraft while enroute does not mean they can descend straight away.
Until the aircraft is established on the procedre then any levels approprioate to that procedure do not apply.