prim2... Your question has been dealt with on here many, many times. It's a peculiar UK procedure to ensure saftey. There are a lot of movements going on underneath ILS tracks into major airports and several serious incidents have occurred due to pilots descending to low altitude when "cleared for the ILS". This has resulted in large aircraft flying at low level over London - I kid you not!. I understand that discussions are taking place to change the procedure......
I doubt if the R/T is often so busy that pilots cannot get descent - I did the Heathrow Final Director task for 31 years and don't recall that happening too often.