I don't know what the legal issues are on this. I don't see why you should have to pay any charges on it at all personally, if there is some sort of charge then I feel Maxtor should pay it. Your drive was still under guarantee I presume and therefore you are covered as far as I am concerned. I would contact Maxtor by email and explain to them you have this charge and see no reason you should have to pay it as you bought the original product in the UK and its not your fault they chose to send you a replacement from overseas. I can't see they have a leg to stand on as you're not going to say that they have no equivalent drives actually in the UK are you?
I did have a faulty drive that had to go back to Holland for repair or replacement once. It was picked up by carrier from my house and delivered to Holland, a new one was sent to me again at no cost and there were no customs charges or anything like it. Perhaps I was lucky?