This is why UK radar ATC no longer use the "cleared for the approach" phrase - they don't like the idea of pilots "doing their own thing" i.e. descending to the platform altitude. I read this explanation in a CAA brochure lying on the coffee table at CAA at Gatwick while waiting for my Class 1, so it has to be right
What you get here in the UK are explicit descent instructions, all the way down to the platform, and then a "turn left/right, base leg" and then you report "localiser established". What happens after that tends to be "[descend with the glideslope] contact Tower XXX.XX" where the [] seems optional.
Essentially correct but the main reason is most likely other traffic operating under the ILS.
Certainly where I work there is a myriad of stuff going on under the ILS and traffic descending at the wrong time to the 2,500ft IAP start altitude would be very embarrassing to all concerned.
There is phraseology that can be used whilst one is on the closing heading, prior to LOC intercept, to give conditional descent with the GP, "when established on the localiser, descend on the glidepath..." and it should be used whenever possible to avoid the late descent scenario.
Just as a side note, in the USA (where I have most IFR experience) it is actually illegal for ATC to vector you through the localiser without telling you that you will pass through it.
You should also be told in the UK if you're being vectored through the LOC, along with the reason why.