A man after my own heart! I am in love with the .303 British too...despite not owning a gun in the cartridge
You are describing exactly what I am looking for in a double rifle, which means SxS to me. Everyone who is making small bore doubles are doing so with ridiculously short barrels. I am also looking for 26" barrels. That length is plenty handy in the woods without sacrificing round performance and accuracy.
If not the .303, I am fine with the 7x57 and the idea of a .30-30 sure is interesting, especially because I have a surplus of .30-30 right now. I saw some a double online in .357 Maximum that was built on a .410 shotgun frame, but it was in real rough shape so I had to pass. Can you imagine going after jackrabbits with that thing! I swear I also saw one once in .221 Fireball but I cannot for the life of me find another example of it.
If you really
really really want a .303 British SxS rifle, I am sure Perugini & Visini will build you own, but it might cost an arm and a leg and another leg. I remember seeing a .458 Lott double they built that was a work of art and I wanted it so badly but you all talked me out of it! Damn you!