I bought from the Japanese auction site Buyee twice. The auction house rules and UK duties are not connected. You pay whatever fees the auction house advertise and when the watch eventually arrives in the UK HMRC will charge 20% of the declared value in VAT and you will also pay a couriers fee, though no more than £20.
Some auction houses may offer a duties included service like the e-bay global shipping program, but those are usually more expensive as they include a brokers fee built in.
Just check what the auction house charge, then add 20% on and that should make you weep