The Brits used mercenaries–most notably from India and Nepal–for a couple of centuries.
But they officered the native regiments with younger sons of the landed gentry and minor nobility and kept them out of the UK.
So they hired the guys as individuals, put them together with their ethnic buddies, and controlled them from Britain.
They didn’t hire armies as going concerns.
John Masters’ “Bugles and A Tiger”, about his experiences as a pre-war Gurkha officer, gives a good view of late Imperial military arrangements.