Duke, it looks like the vendor in England offers maybe a combination muzzleloader. Look at item #1154C, Kodiak Combination .50/ 12ga. It looks to be a lot of money.
Take a look at Cabelas. They sell the Pedersoli shotgun in 10, 12 and 20 gauge. The shotgun is a double barrel. The prices range from $729-934.99 just for the shotgun. The prices range from $769-974.99 for a shotgun and a starter kit.
A 20 gauge is approx .60 cal. A 12 gauge is approx .72 cal.
For a modern shotgun, I think you'll find the 12 gauge is the most common.
I have a Pedersoli 12 gauge, double barrel, and am happy with it. My brother also has a 12 gauge muzzleloader. He shoots birdshot and ball out of it.
You mentioned pheasant hunting. Where we live, you can't shoot pheasants unless they are in the air and flying. "Roosting" as you put it would imply the birds might be sitting up on a tree branch. I can assure you they do.
I have taken a ton of pheasants. For me, the 12 gauge is perfect. My shotgun has screw in chokes and I can reach out pretty far with the shot pattern. Thirty to a little over forty yards is about the nomal range for taking the pheasants.
You need the energy and more shot to take the pheasant, a 12 gauge will deliver that. Otherwise you risk wounding the bird and losing it by using something smaller.
For target shooting you might consider a rifle, and that is a whole other animal.
Do take a look at Cabelas sporting goods. It will show you photos of the various muzzleloaders they offer. And in passing, the prices you pay over there seem to be pretty close to what we might pay here if I did my math correct. :thumbsup: