I often find the charity shops are a good source of DVDs that i might not even know existed, I recently found a copy -English soundtack - Chimes At Midnight , which has only ever been released on disc in Spain, for a pound.
The big charity shops - that is the chains - like Scope, Heart, Oxfam seem to overprice almost everything basing their DVD prices on Amazon. These chains i suspect like the BBC, The Police , Army procurment departments are sort of taken over and run as much to benefit the people who operate them.
The good deals are always in those small stand alone sort of charity shops which are seeking to help suffers of something or other that i have never heard of, or a special service ie Celebrate which runs a mobil sex toy library for the housebound. The local hospice ones also seem to sell DVDs a low prices and for some reason have a better selection of the more intelligent films. Especially good is the one called -fast track departure - which has some arrangment with a Swiss travel company.