Britain ceased being an imperial state a long time ago, but it is better to be a sovereign, independent minor power than a mere province in a mighty empire. Imperial subjects, even those who live in the ‘core’ state, rarely have much power over their own governments and Disraeli-style talk of dazzling imperial grandure and power on the world stage is a way of distracting the ordinary people from this fact.
Britain ceased being an imperial state in reality but in mentality it remained an imperial state. That was apparent also in the negotiations.
As to being a sovereign minor power - yes, that’s fine, but it would mean giving up the earliest “colonies” like Scotland and maybe even Wales (though I doubt the latter)
You are correct in a way about how different parts of the country fared under the British Empire, but it is no doubt that hte UK as a whole turned from a gin-soaked, second rate, poor country in the early 1600s into a powerful super-state by 1800