DZ's have there own common frequency, 129.9 and all drops are called two minutes ready to drop and drop complete descending.
There is no policy for having props stopped when there are canopies in the air, if we did that we would never get any work done. We asking visiting private pilots to do so as its easier than worrying what they are doing as they don't know where to look for the canopies, we do.
Pretty much every DZ I know of also works a local FIS/RADAR unit and the calls are also made to them on the frequency. Short of announcing on London Info which would make the frequency rather more busy than it already is and would be highly confusing when you consider the sheer volume of DZ traffic and the fact the London Info is rebroadcast across the entire frequency there is not much more we can do!!
It's not difficult, read the map, talk to the DZ or the local radar/FIS unit and keep out the way!