Britain won’t regain the preeminence it once enjoyed, neither will the USA. The USA’s bizarre and counter productive military efforts beginning with Vietnam have served to hasten its own decline. Ceaseless repetition of the same actions with the expectation of differing results is not rational.
Both countries would do well to cease chasing the divergent and irrational dreams of global hegemony not called hegemony, and free trade that is not free. They should combine with each other and other closely related countries to regain hegemony over their own domestic economies.
The militarization of foreign policy will lead to further economic decline and finally military defeat. The truth is neither country faces grave external foes, they are primarily domestic.