I don't know if it works in UK, but I used to do this for suspicious seller.
I would ask the seller to take a picture of the authenticity card, the caseback with serial number, with a piece of paper showing his id.
It proves that he indeed has the watch.
If everything is fine, I would pay by pp with American Express. I would request a next day delivery. If I receive a replica, I would file chargeback with American Express immediately.
The worst case is that pp will close your account as you violated their T&C. But You will get your money back from Amex.
Btw, I wouldn't buy the watch if I can't go to pick it up. It's too much risk involved. You are just asking for trouble to save some money.
Not worth it imo. From the pictures, you can see that the seller knows about watch. He features the luminous effect, and the regulator.
The funny thing is that he doesn't have any descriptions. I would suspect the pictures are stolen from somewhere else.
my 2 cents.