The usual suspects are Holts and Wallis & Wallis, but as you know with auctions you are at the whim of the buyers on the day and their commission/VAT on commission. Worth googling to see if you have any auction houses near you who cover Arms & Militaria.
There will be people saying you’re best off trying to advertise on here, but it depends on the collection and whether you can/wish to catalogue them yourself. In which case lots of good/high-res photos and honest descriptions are essential. If you have a few pistols of particular interest/value you could always try on here first with them.
Of course ultimately it’s your choice.
Best of luck.