This is done on a regular basis out here in Arizona and works like a charm
I'm afraid this is a syptom of the UK, where often people don't bother talking to anyone. I would have assumed the drop plane should be in contact with a radar facility who can advise them on traffic, whether they are chatting to them or not. It would be good airmanship in my book.
In the USA they have a frequency for drop zones so you can directly call up the drop aircraft and ask for advisories on the drop. Makes sense to me.....
EA