That's 455 Webley, not 450 Bulldog. You can shoot 450 in a 455, but not the other way round. The Bulldog cartridge is the American version of 450 Adams:
http://en.wikipedia.org/wiki/.450_Adams
DO NOT SHOOT 455 IN A 450 REVOLVER UNLESS YOU WANT SERIOUS INJURIES!!
Furthermore, 450 is a black powder round. Most, if not all, revolvers chambered in 450 are BLACK POWDER ONLY. Please be very careful and do some thorough research. They are many identical revolvers, there's only one of you.
I think you can only handload 450 Adams now. You may be able to find it from some commercial reloaders, but that's it.