O.K. Your gun is chambered for the .455 Eley, or .455 Mk. VI cartridge, not the .45 Colt cartridge. The .455 British cartridge is much shorter than the American .45 Colt round. All of the markings indicate that it was a revolver purchased by England during World War I. The marks you show are all British acceptance marks or approval by different agencies. There is, or was, a book telling the full story of British proof marks.
I don't know where you live, but .455 caliber cartridges, usually found under the name .455 Webley, are very hard to find, usually available only through specialty dealers.
I once had such a New Service that had been issued to the Royal Canadian Mounted Police, also in .455 Eley caliber. However, my gun had been reworked to accept .45 Colt cartridges, a fairly simple conversion. There is a slight difference in bore diameters, but not enough to make any practical difference.
As to value as is, probably closer to the $400~$600 range at the very least.
Hope this helps.
P.S. .45 Colt is the actual designation of a specific cartridge, and there are several, not all interchangable. The brand name would be Remington, Winchester, Federal, or something like that. As to .45 cartridges, the more readily available are .45 Colt, .45 ACP (.45 Automatic), or in some cases .45 Smith & Wesson. The .45 S&W is also called the .45 Schofield now by Cowboy Action Shooters. They are not interchangable.