Six quid for 2 pounds of tripe sounds a bit dear to me. You should look for a slaughterhouse that'll sell you whole unwashed tripes (15 to 25 pound weight usually?) or get to know someone who works at a slaughterhouse.
I buy waste from butchers who slaughter their own animals and from a chicken plant I used to work for. Lamb necks and ribs free, chicken necks 5 euro for 10 kilo box, beef hearts fiver each, pig hearts a euro, minced trims 3 euro a kilo etc. etc. etc.