<< British gun makers could have made any action for their customers. It was the insistence of the customers to have the best handlong, strongest and best balanced action that caused thsoe makers to favour the sidelock. So the sidelock became pre-eminent was because it was the best solution to the customers requirements. >>

The side lock didn't "become pre-eminent". It was around first, all doubles had side locks, only later did the box lock arrive on the scene.