I heard of an incident where in the London TMA a scandinavian a/c was cleared to establish ILS while descending to 4,000 alt. The ATCO expected the a/c to maintain 4,000 and report loc established, however the crew descended to a lower alt (without direct clearance to do so) and "hunted" the loc beam AND the glidepath from the lower alt. I believe a proximity problem with another a/c resulted.
The MATS Pt 2 says that the specific clearance LTMA controllers give is to intercept localiser only as a method of ensuring the a/c descends to the specifically cleared altitude.
Remember in the London TMA, ATCOs operating under the privileges of an area "ticket" are allowed to clear a/c to (1) a minimum of 4,000 and (2) to intercept the localiser ONLY.
Maybe, because of different airline SOPs, the safest way of ensuring that an a/c does not descend below its cleared level is to clear it to est loc only !!
Just my viewpoint as an LTMA person, so there maybe some approach folks who can give a more enlightened argument !