Alex.. This has been dealt with many, many times on here. Underneath the approaches to some UK airports there is other traffic - maybe landing at other airfields/heliports or maybe just transitting... light aircraft, helicopters, etc. We found that some pilots, when "cleared for the ILS" descended to very low level in direct conflict with traffic underneath them. As a result, we adopted the procedure to establish on the localiser. When thus established they were told to descend on the ILS - that is, follow the ILS. It's all to do with safety, which we're super conscious about over here. It's no big deal; it happens thousands of times every day but just occasionally a new pilot becomes curious.