I wrote about this a bit ago. Daniel Hannan, Conservative Party MEP for South East England, has a more recent thought.
Churchill [as early as 1946] makes clear that this United States of Europe should not include Britain:
There is already a natural grouping in the Western Hemisphere. We British have our own Commonwealth of Nations. Why should there not be a European group which could give a sense of enlarged patriotism and common citizenship to the distracted peoples of this turbulent and mighty continent and why should it not take its rightful place with other great groupings in shaping the destinies of men?