There were several Webley revolvers chambered for the .450 Revolver round. It's an old blackpowder round, hasn't been around for a long time.
Would need to see a photo for best identification. Some old Royal Irish Constabulary Webleys were sold in .450, also some very cheap copies were made, both in Europe and the U.S. Maybe some H&R revolvers were made in the caliber, all under the name of "Bulldog" or British Bulldog."
Is your gun indeed made by Webley?
Either way, the gun, in good condition, could be made into a shooter if carefully done. Ammunition would have to be fabricated, probably using .45 S&W brass, which is available again for Cowboy Action shooting. Also, black powder only should be used to maintain safe pressures.
Would like to see a photo.
Sorry, as to your last question of value~again need definite identification. Could be $100 for a copy, $300~$450 fro a good Webley.