The Postal Services Act 2000, states that:
(1) A person commits an offence if, without reasonable excuse, he -
(a) intentionally delays or opens a postal packet in the course of its transmission by post.
So, if the address on the parcel is valid, it would seem to me that you can 1. get your postie to take it back or 2. take it yourself to the nearest Post Office for it to be redirected or you can 3. take it to the proper address, yourself (especially if you choose the second option and the nearest PO is that side of the county. Or 4. you could contact the recipient and ask them to come and collect it.