Was it a mistake for the british to use mercenaries to fight the war? why or why not

In any war where mercenaries are used, the nation who employs them risks many things. Some are of ethical and moral nature, other are about efficiency. Mercenary armies have been present since ancient times and the british are no exemption to using mercenaries when fighting any bigger war (you didn't really list which war you meant). Mercenaries were usually viewed as less moral and ethical soldiers as they didn't care for the reputation of their employer, while soldiers fighting under a banner had to. At the same time, they often provided the extra edge to a fighting force which enabled them to win.