I bought one from Honk Kong before Christmas. You pay in full, but it is put in an escrow account until you receive the watch. I'm still waiting with current delays at present. Once you have the watch you confirm receipt and then agree for the money to be released to the seller - so no risk.
Expect to pay 20% VAT and £12 couriers fees, but a few parcels have gone through without charges being raised of late, but you cannot guarantee that so factor the 20% in.