Nice video by LifeSizePotato's channel, drawn from his eclectic gun collection. It's a quick way to determine if your Webley's cylinder has been cut down and modified for .45 auto and moon clips in place of the rather harder to get (in the states) .455 Webley. A lot of the guns here have been modified, but most Webleys really aren't strong enough to plow away on off the shelf 230 grain 45's. The one's built since 1915 can take it in my opinion, as their cylinders were beefed up for nitro powder and are pretty strong.
What can you do to scratch your Martini Henry itch at International Military Antiques? Looks like you can do a lot. First off you can buy a Martini Henry from the Nepalese hoard at pretty reasonable prices. A cleaned up nice long lever Martini is about 600 bucks. (Really should get it checked out by a competent gunsmith before entertaining any notions of firing it.) And then if you have got it checked out and have the go ahead to shoot it, you can get some nice fat .450/.577 ammunition to feed it.
Of course, then you'll need some accessories. They have a pretty nice bandolier to put all those cartridges into, and it is quite handsome at that. The other leather thing you'll need is a reproduction guard to go around the barrel to protect yourself from heat from repeated firing. Just like the real thing, which have all disappeared. Leather goods don't seem to last forever...
So then what do you do when the rifle breaks down. It is a machine after all and machines break sooner or later, although guns seem to last a lot longer than cars. IMA has you covered there too, they have plenty of parts. This will probably test your gunsmithing skills, but learning new things is always a good thing. Below is a trigger spring, striker spring, and a spare striker, too. Enjoy!