I had one that was converted to use .45ACP from moon clips. It was a cool pistol, and it worked fine, but the sights were a bit tougher to use than, say my Sig P220. It was one fast reloading revolver though. Break it open, the empties fly out, the full-moon clip went right in, and you shut the gun. Faster than a swing out revolver. About the same speed as a .45 Auto.
As for value, I paid $150 for mine. That was over five years ago. I'd say due to the large number of .45ACP conversions made in the post-war years by people buying them as surplus, that original .455 are probably more collector oriented. It is definately more market driven than anything else. They made them a good long time, and I don't know enough about variations to tell you what you have, but sites like GunsAmerica.com could probably give you a ballpark.
It was the standard British sidearm for a very long time. So they were made in large numbers, as their service time was in the heyday of the Empire. They reached every corner of the globe because of this. They are good quality guns, but in the US they aren't valued by non-collectors too much. I sold mine because I didn't have enough money back in those days, but I'd buy another one these days to play with.
Ross