The Britts would probably speak a Romance language instead of a celto-germanic one.
Celto-Frisic, to be exact. I'm from Terrapinia (OOC: North America), Kanata to be exact, and our educational system doesn't quite group all of the 'Germanic' languages together as they would in Greece, Russia, Tejas, Aururia(OOC: Australia), and Japan, and as it used to be in Francia before the Great European War(hatred for most non-Nordic ethnicities traditionally labelled as Germanic continues to be widespread over there, sadly, with the Frisians, Dutch, and Belgians being the only exception, and Belgians themselves are quite the mix).
Rather, according to the Kanatan school of linguistics, there are four main groups: Nordic, Teutonic, Austrian, and Gothic, with the latter two partly isolated these days(Frisian and Dutch are in their own category, and where a key building block of today's British, which is considered to be it's own language).