Run as fast as you can...
Also, don't worry about eBay with not paying. As a buyer, he can't leave negative feedback. Open a dispute thing and say you no longer want to purchase the item, which you have every right to do for no reason whatsoever.
Next, he'll complain. You email him and blackmail him, say you will email eBay the email he sent you about paying in cash (which is not allowed) unless he agrees to cancel the transaction.
UK law has no bearing whatsoever on an eBay transaction. Unless the watch is not a watch but a spare kidney You are totally in the clear mate, don't stress, walk away, there are millions more watches to spend money on