Cheers for the replies. The example was purely a fictional one for the sake of argument, and in fact a more realistic case is the one in Lock n' Load's reply.
It's not an issue in the UK as such as there is no significant terrain, but what prompted my question was going into say the major towns of N. Italy, ie just beyond the Alps. There, like in Calgary's case, I'm pretty sure you get a descent clearance to a lower altitude when you're still in the higher vectoring area, but it is of course obvious that you are not going to hit any mountains as they're behind you at this stage.
The kind of anticipation I was talking about was precisely the type described by L'nL - but are the rules different in the UK vs in more hilly countries?
The example I'm thinking of is the arrival on to 23 in GLA where you get held at 3000ft on base because of the hill on the northern side of the extended CL. You only get descent to 2300ft (?) when you're on the final intercept heading and presumably in the lower vectoring area. This approach leaves you momentarily above the glide - which is not a problem, but requires a tiny bit of forethought when considering your speed and config particularly in the case of a tailwind.
As the a/c commander (or his 'assistant', in my case) is the one legally responsible for terrain clearance even when under positive radar control, I just wanted to get a bit of clarification as to what the rules are for you guys - for the next time when we're descending in IMC!
p.s. The Spaniards get round the problem by asking "are you in visual contact with the ground" when descending you into Barcelona. Again, fair enough.