Just checked out the Gamebore Bismuth shotshell offerings online.
Looks like the cartridges for pheasants etc are simply called "Bismuth HD", for high density (specific gravity 9.6 they say),
2 1/2 inch shells available in sizes 4 and 5 (UK sizes I assume).

Click HERE to view the webpage.

They also list a "Dark Storm" bismuth cartridge for waterfowl, that is a 2 3/4 inch shell in sizes 3 and 4.

Apologies for delay in tossing up the "Make Your Own Bismuth Shot" article from Guns'n'Game magazine: under the pump at the moment.