Britain was mostly able to fight because the French fleet was sunk which shielded them from a massive invasion and Russia weakened France. By themselves they wouldn’t have done much.
And, well, most of the Coalitions were almost exclusively funded by generous British subsidies. To Austria, Prussia, Sweden, Russia, and a couple dozen of minor powers. Industry made Britain able to afford that.
"In terms of soldiers the French numerical advantage was offset by British subsidies that paid for a large proportion of the Austrian and Russian soldiers, peaking at about 450,000 in 1813."
