Cancel the order citing.... Problem with Buyer's Address.
To sell only to mainland UK , you need to show this in your Site Preferences , there is also a section to tick if you do not wish to sell to places like the Isle of Man , Channel Isles, Highlands and Islands.
Though be aware, for those places there is no extra postage cost if you use Royal Mail. With RM it would cost you the same to post to Jersey say, as it would to your next door neighbour.
Check those countries in your Site Preferences on your homepage here:
https://www.ebay.co.uk/ship/prf
On the left sidebar select Postage Preferences, scroll down to Exclude Postage locations and select the countries there you do and do not wish to post to. There is a little box at the bottom of that list to add this to any item listed , with no bids.
@dinglewoody