Without the income from junk mail contracts, door-to-door Royal Mail deliveries would cease, at least in rural areas. At the same time the Royal Mail have failed to cash in on the boom in internet-ordered parcel delivery thanks to an insanely long list of prohibited contents. Letter delivery could be combined with parcel delivery, saving costs, if they could get their act together. The PO are looking like BR wagonload all over again.
As for the junk mail, firstly I post back any pre-paid envelopes (stuffed with the rest of the junk mail). Sometimes I fill in any forms requesting more info and (if it's an option) for a rep to phone or drop in, giving the email, address and phone number of a spamming company (find them by searching for "Tailored Marketing" or some such BS). Anything left over I use as kindling to light my wood burner or garden bonfires.
Anyway, it seems to me that the volume of junk mail is much less than it used to be, with the advertising focus having moved to the internet these days.