Thats gas that they want to charge you, that stuff is like you say, worth a lot of money. I work in a, ahem, waste management related area, and would bump into a lot of travellers who would give you a few bob for them, either that or just take them off your hands at least, don't know if they'd be used in making art though.
Any scrap yard should buy them for you, friend of mine recently went with a few bits of copper piping and an old cylinder, and got 50E for her trouble.
Midland Scrap metal in Mountmellick would be a good bet. No relation.