Britain would probably have about one constituency for the euro
Which one?
Something in SW London, probably more than one. Richmond Park, Twickenham, etc. Also Hampstead and Kilburn, Hornsey and Wood Green.
Also a significant chunk of Scotland, including most or all SNP-held seats and maybe some of the rural LD ones.
The Guardianista seats in London I might give you, but definitely not the rural Lib Dem areas, which they win by running to the right of Labour. For example, UKIP gets some of its best scores in Cornwall, where the Lib Dems had all five seats before May.