Frank,
The Society can help you with a Booker card, however you'll have to remember you need decent storage space to buy from wholesalers. For example the ribs Robert mentioned above come in 10KG boxes so that's about 5 untrimmed racks which requires a bit of freezer space
The prices you got from the Supermarket are not too bad apart from the ribs which I'd say are bit pricey as you mentioned also. Supermarket meat is ok, particularly if you're only cooking for a small family or just a couple, it's cheap and the quality is normally acceptable, quite good sometimes even, but it can vary quite a bit. There will be plenty of people on here who will have found your post useful though, so thanks for that!
Personally I don't buy meat from the supermarket because I can get much better meat from my local butcher. The problem with high street butchers is that the price point is generally higher, but if you get friendly with your butcher, discuss what you do on hte BBQ, even bring him in a few samples you might find you get more "special" deals. You'll most likely find he ensures you always get choice cuts too, unless he's a w**ker of course in which case find a new butcher
My butcher Jennings, in Caversham, Reading are absolute stars, they hold nice pieces of meat in for me, they age my brisket, they even sharpen my butchery knives for me. I can't discuss the prices I get as they sponsor my competition team so I get special prices, but their general prices aren't uncompetitive they can always tell me exactly where my meat came from and exactly when it was killed. There's still plenty of good Butchers up and down the country and we need to support these guys before the supermarkets and wholesalers destroy them.