I never quite bought how three million Limeys could be ferried the length of Atlantic and more, while so many fewer could cross the Mediterranean from France, Spain, and Italy. Also the collapse of California is doubtful and the effects of tidal waves are probably overstated.
Actually, I'd expect Spain and Italy to have remnants remaining, perhaps allying with France, or maybe even merging with it...
Relocating the Spanish capitol to Seville and the Italian one to Naples (at least temporarily) shouldn't be too much of a strech...