Do you have a polyester Capilene® baselayer from Patagonia? Is it worn out? Is it really worn out? No, can it still be worn as clothing? No? Well, if it's not made of nylon (which poses recycling issues, so the program is restricted to polyester Capilene only - they promise they'll expand the program as their technologies develop) and it's from Patagonia, well, ah, when you're done with it, they'd like it back.
Patagonia will magically use "fiber to fiber" recycling processes to "make new Capilene garments from old". They say "Our research shows the environmental benefits of recycling Capilene fabric into new clothing are significant." And then your pants live on to warm some other lucky human, perhaps a supermodel or Christian Bale. But they can't possibly stress enough: "We don't want your Capilene baselayer if it can still be worn. But if it's so worked no one would ever wear it again, give it to us. We know what to do with it."
You can either drop it at any Patagonia store (and they have a list of UK cities) or you can mail it here:
ATTN: Common Threads Recycling Program
8550 White Fir Street,
Reno, NV 89523-8939