Breechloaders were a royal pain to manufacture en masse in those days. The precise tolerances required were very difficult to do.
The US Hall rifle was a strange design (you muzzle-loaded the breech, if that makes any sense), an was doable in the 1820s. You note, however, that even that simple design didn't survive - the ACW was fought with muzzle-loaders.
Could they have been adopted for e.g. skirmishers, where the cost wouldn't have been quite so much of an issue? possibly. For the whole army? much harder.
Heh. One answer might be if the British Army was 1/10th the size of OTL
