As far as I am aware the "Postal Preference Service" is, as you say, a scam designed to find out what sort of junk mail you would prefer (in terms of what sort of products you might buy) and then to make certain that you get plenty of it. There is another service called the "Mail Preference Service" which is run by the Direct Marketing Association and which says its idea is to stop DMA members wasting their money on mail which goes straight into the bin. I registered for it about a year ago, but I can't say I've noticed much difference in the amount of c**p coming through the letterbox. Now the Telephone Preference Service is another matter. I registered with it about three years ago and haven't had a single call since from anyone selling double glazing or anything else.