I think you can buy 455 Webley cases which I think is the same thing from Graff & Sons and that they are made by Hornady. I would not mess with changing it at all as it will wreck the value of the pistol and for what it was made for 455 Webley is fully capable. Cheap Lee Hand press about 30 or 40 dollars and a set of Lee Dies about the same. Entertaining cartridge and really a lot of fun in an old Webley. Probably lots of fun in your pistol too.
I bought cases from Graff a few months ago and Webley loads are mild so the brass will last about forever. I don't have any experience with the 455 S&W or Colt pistols, but the 455 in a Webley likes soft boolits pushed slowly. Think mild Unique loads. A vendor here (Matt's Bullets) sells the right boolit for the round.