There's a auction house in witney oxfordshire

http://www.woma.co.uk

most of the police stuff goes through on a tuesday - i've been on a police night and they are cheaper. on the plus side all the cars are well cared for mechanically but some of them have scuffs & scrapes etc

cant really help with price info as it varies, but look at fourth coming auctions. the list is normally there a few days before