I've been a Holt's customer for about 20 years now. I would warn readers that their auction estimates are usually half of the hammer price. They keep the estimates low to tease out maximum interest. Add to that, then you have the hammer premium. Then you pay 20+% VAT on all of it. Then you delude yourself that you should get the VAT refunded but realize its so much effort, you don't waste your time. Then you have storage. Freight forward of firearms inside of the UK. Then international shipping. Then you have BATF importation and engraving of importer marks. Then freight forwarding to your FFL. Then your normal transfer fee. Oh, and you have international wire fees, plus you have foreign transaction fees and then credit card processing surcharges.
I strongly recommend paying 250 pounds to have a gun evaluator look at the weapon you'll be bidding on before you decide to bid.
The best rough estimate of what a gun in the UK at auction costs you as an American is taking the hammer price with buyer's premium in UK Pounds, double it, and that is roughly the landed price in US dollars.
You really don't go to the auction houses looking for "deals" because you're the dumbest sucker on the planet anytime you're the highest bidder on the best advertised gun sale of the year. You buy at auction because you're looking for a Holy Grail - Unicorn that you want at any price and exists at no price anywhere else on the planet.