Pyrodex can work, but has considerably poorer ignition properties than black. Same goes for 777
Neither worked at all well in my Tryon, never mind the other downsides, trying to spark off substantial amounts, with a no.11 cap. My mentor in learning to muzzle load at the club (to get the tick in the box for my safe shooter's card) was a European champion in muzzle loading rifle competition, sponsored by Pedersoli as it happens. And this was his old rifle. Useless in his flintlocks also, we discovered. I only bought the one tub as an experiment so can't claim any wider experience. Other club members didn't like it either, definitely a lesser experience than black.
We got it going properly by putting in a pinch of black before the main charge. I think this is what they also do for the pellets, they have a little bit of black an the bases. Not that we use pellets. Seemed to work ok in e.g. muzzle loading revolvers where the quantities are much smaller.
It's only advantage is that it is still classified as a propellant, so less hassle to store. Otherwise it is just black powder, sulphurous, reformulated with inhibitors and potassium perchlorate as an oxidiser
Black is not difficult to find here, if your dealer doesn't have some, they can get it in or you can order it for them to hold for you. Henry Krank are the specialists and hold a wide variety. British, Swiss and Polish.
https://www.henrykrank.com/reloading/powder/swiss-black-powder/
777 was also classified as a propellant, and IMO much nicer.no sulphur in it, less fouling too. But has now been reclassified as an explosive, so the same rules now apply as to acquiring and storing black. Not onerous, the license to do so is free, just that we have to keep it in a wooden compartmented box to a certain specification. We are allowed to keep up to 10kg of it in our houses.
This is the one I have, Peter Starley sells them at cost price to him, to give you an idea:
Storage Boxes - Black Powder ISTR I paid about £30 for mine, years ago, thought of making my own, but it wasn't worth it.
So really, for me anyway, I just use black, or 777 for undemanding stuff like pistols.
Hodgdon triple seven powder and pellets are not exempt under Schedule 2 of the Explosive Regulations.
This means that any registered firearm dealer selling this product needs to ensure that they have an Explosive Acquire and Keep (A&K) certificate with the UN numbers of 0499 or 0501 listed.
If you already have this certificate but do not have the UN numbers listed on it, please return your Acquire and Keep certificate to the Firearms and Explosives Licensing Unit for amendment. There is no fee for this work.
For any customer purchasing Hodgdon triple seven powder and pellets, please ensure that you have a valid Explosives Acquire and Keep (A&K) certificate with the UN0499 and 0501 listed on it, which the registered firearm dealer will check. If you do not have an A&K certificate please apply to your local police force.
For anyone who already has an A&K certificate, please return your certificate for amendment, detailing why you have returned it.
You can get more information on the government website
Black powder substitute - Wikipedia
I suspect that trying to run an 1859 percussion Sharps with tube cartridges, on Pyrodex, might become an exercise in frustration., hangfires at the least, but maybe the musket caps could overcome that issue.
Still thinking about the I,A.B, but could see it becoming quite a project, and I don't need any more projects at the moment, fun though it might seem.