The only stuff I've found (for a different model Webley, but same caliber) is made by Fiocchi, but it is very expensive. $46.95 per box of 50 if I recall, from another vendor. Marstar doesn't carry it (at least not recently). One or two other vendors had it at an even higher price.
New brass is kind of scarce as well. And be advised there are 2 lengths of .455 Webley brass for the 2 cartridge 'Marks', MkI and MkII, not to be confused with the Mk numbers of the actual revolver models. MkI is longer, MkII is shorter. There's about 3 or so different makes of more common brass you might be able to cut down to make .455 Webley, but, in addition to trimming the case walls down shorter, you also have to trim their rim thickness back, from the front to the back, to match the Webley spec. And each one of those other calibers has a different rim thickness, overall length and inner volume, so pick carefully which one you're going to use if you go that route. I bought some new Bertram brass a while ago to avoid that, but it's also horrendously expensive.
And then there is the vintage Webley lead weird RN bullet profile and weight which is not very common to get hold of either (if you're interested in reproducing it exactly). There's another website dedicated to stuff about these vintage arms, I forget it's name right now. But that's where there's a few real gurus on it. I think some of them are re-enactors or something as well.
Anyway, it ain't easy or cheap to shoot a Webley.
Pip, pip, cheerio.