How plausible is it that, in a last ditch effort to save their empire, the British decide to federate the Commonwealth/Empire based off of the Canadian or Australian model?
It's not difficult to imagine free trade area etc EU styled but not one superstate; they would need independance and home rule rather than rights as a state. Read Rule Britannia
Not at its proposed founding (1910's-1920's) Britain has the factories and the colonies the raw materials, if Britain then uses the wealth to streamline the Commonwealth it could work