An outfit called Peter Dyson used to sell BP shells. I dont know the practicalities of sending them across the water though. If you are looking for 2 1/2" light loads there are many available, I use Eley Impax 1 ounce a lot, not sure of velocity etyc as I am in the Med just now. Eley "First" also have a 1 ounce load in 2 1/2" case and something like 7/8 ounce also. I think Hull Cartridge etc do similiar loads.
I don't know that there are any praqctical advantages to using BP loads in a damascus (BP proofed) gun as opposed to a light load nitro. Main thing for me is to check bore diameter against proof marks and just as important, remaining wall thickness.