What browser are you using? Try another to see if that makes any difference.
What happens when you try to add or delete an item from your Watch List?
I just went to ebay.co.uk and tested deleting an item from my Watch List and adding it back and did not have any issues.
You can also try deleting items from the item page (unless it is out of stock), or from the Summary page. Or using the eBay app.
Some Watch List issues can be helped by signing out of eBay and signing back in -- you may want to do that in any case, since you are currently on ebay.com. Sign out here and sign back in to ebay.co.uk.
Earlier today there were some site issues which may have sent you to the US site by mistake.
