UK was the leading light in proof laws and proof houses. There is no difference in original quality between Brit and Belgian barrels, comparing grade to like grade. I'm not aware of any brazed flint or percussion barrels - maybe so, but I don't know about it. Properly tinned barrels are very corrosion resistant. Soldering is a perfectly fine and strong enough way to join barrels. If/when the joint fails, it is reasonably easy to repair. It can be regulated if the barrels don't shoot together. Brazing lends itself more to production line work (oven brazed) than to hand work.
Sound barrels will be the key to a suitable percussion gun. Springs, screws, and cracked wood are the most likely sources of repair needs. Locks can usually be rebuilt relatively easily via micro welding.
