<<I was told, a long time ago at CATC methinks, that there had been a few incidents involving pilots given "cleared ILS approach" and then immediately descending to MDH/MDA regardless of step-down fixes or traffic beneath the glide,>>
Yep - quite true. I was on watch at Heathrow when an a/c given "cleared for the ILS" went straight down to 1200 feet over Central London. In the case of Heathrow there is traffic underneath the ILS heading for London City Airport and also a great deal of helicopter and light, fixed-wing activity. Procedures ensure that inbound traffic to Heathrow following the ILS GP will remain separated from that traffic. At Heathrow you should get descent clearance in time to stay on the GP (with any luck!).