Does it say on the listing where you bought the item, location in the UK, if so there address in the UK is where you return to.
BUT you return to the address ebay provides when you have opened a return case. Returning to any other address will not get you a refund, as you need proof of tracked delivery to the address provided by ebay.
Have you even opened a return case? You must do returns through the ebay system, money back guarantee is on every listing, right hand side.