Q1 - Dunno but I hear there is a review going on to try and harmonise the different TAs in the UK ... but no doubt they can't agree whose to use!
Q2 - I believe it stems from a number of incidents a few years back when the aircraft was put on a closing heading at, say, 4000ft when the published ILS procedure started at 2500ft. I think several aircraft took a clearance for the ILS approach as clearance to descend to 2500 immediately. As HD says, it's not uncommon to have traffic going under the approaches (often VFR so it doesn't just apply to LHR) - not a problem if the controller has calculated that the ILS traffic will intercept the glidepath well past the transit traffic at its last cleared level but a bit of a shock for all concerned if the ILS aircraft drops earlier!
On the other hand I might be completely wrong. Why should there be any logic to aviation?