If you want to buy a good amount of meat for a good price in London try new covent garden market on a Sunday.
Thorne & Sons are a hybrid between auction, showmen and butchers. With a microphone, and tens of kilograms of meat they will sell you £20 of meat. Get there early and look undecided as they offer you some ribs, saussages, rib-eye, lamb, chops, etc ... the whole package for only £20. Be prepared to freeze or to have a big BBQ.