Butane is supplied with those indoor room heaters and barbeques. They often only get used a few times before they are left out for scrap with a best part of full bottle. At least, thats the case round here! Like AW ive got a pile of them now and have given a couple away.
Ive found a good use for them is refilling the 907 camping gaz bottles that all the T5 owners seem to use. They readily pay £10 or even £15 for a full one (approx 2.5 kilo of gas!)
Energas dont require a deposit or exchange bottle for their 19 and 47 bottles if you have a branch near you.