Ok, I've got an ever better gun candidate for you than the revolver I posted earlier.
This is a Colt Model 1903 .32 Automatic that was engraved and gold plated at the Colt Factory when it was manufactured around 1926.
Colt records indicate that only 27 guns were factory gold plated and engraved. That makes it rare and expensive and very unique. Further, the description says that "most" of the 27 were shipped outside the U.S. That would explain how he got it in the U.K. (Obviously, he wouldn't be the first owner. If he's not worried about UK gun laws he could have acquired it from the family of the original owner in a private, illegal, sale or even been given it as a gift. Or, heck, maybe he inherited it. If he "comes from money" I could see his Dad or Granddad buying this and passing it down).
A couple notes: The .32 Auto cartridge is a small cartridge designed for small concealable handguns. It doesn't have as much "stopping power" as larger caliber rounds, but will definitely kill someone if you hit them in the right place. This ammo is still being made.
The Colt Model 1903 .32 Automatic Pistol was a very popular design noted for it's reliability, sleek look, and slim design. It was from an era where guns were hand fitted by professionals and an engraved and gold plated example is as fine an example of the gun makers art as you will find.
This particular gun, being so rare, and being Gold plated and engraved, would definitely fit someone who is a bit of a Dandy and who wants to flaunt his wealth. Personally, I think it's perfect for the way you describe the character. It's also still a practical self defense pistol. (Not the best, but workable).
Note that guns like this are typically hoarded by collectors and not shot. But, he's vain enough to just carry it around due to his personal sense of style and not concerned about the "loss of value" from any wear on the gun like a collector would be. He's above such concerns and the money is triffling.
Note the small sights. That's how they were back then and it does make it more difficult to hit the target. That could help explain why he waits until close range and still hits the wrong guy.
Here's a couple links to the description of this gun and another one.
http://www.coltautos.com/cagotm201207.htm
http://www.coltautos.com/cagotm200308.htm