When the Romans came to Britain, the people living there were speaking a vibrant, living Celtic language. After the Romans left, the Anglo-Saxons came to find the same people speaking Latin alongside the now-weakened Celtic language; the situation was probably much the same as it was in Wales. There was so little of the original language left that there was no point in taking any of it over; the natives learned Old English, while the invaders gave their own names to native flora and fauna as well as cities, rivers, and forests.
The Old English language as it was written down at the time was an aristocratic register; the average, everyday English was probably a lot less complicated. The Norman Conquest wiped out the English language amongst the higher classes. It was relegated to the commoners, who spoke a somewhat more Celtic-influenced register. From here, Middle English, with its more analytic grammar and lack of inflections, proceeded to evolve, and then into Modern English.
What if, like in Wales, neither the Romans nor the Anglo-Saxons had as much of an influence as they did on the downfall of Celtic? What if, to this day, a language descended from the parent language of Welsh, Cornish, and Breton were alive in the hills and woods of Lloegyr (England)? This is what that language might have looked like
Alternate history
Loegrish (hinn yeith Legoryg, pronounced [hɪn jɐ͡ɪθ ləˈgɔɚ.ɹəɣ]) is a member of the Brythonic branch of the Celtic languages, spoken natively throughout England, primarily in the central and southern regions, with minorities in the United States, Canada, and the other Celtic nations. The people who speak the language are called the Loegrish people (Legoris, pronounced [ləˈgɔɚ.ɹɨs]). In English, older Loegrish people sometimes refer to it as "Welsh".
According to the United Kingdom Census 2011, only about 1.68% of the population of the United Kingdom speak the language. The vast majority of Loegrish people live in England, but small communities in Scotland have existed since the Welsh Exile in the 16th century. "Welsh" here refers to all Brythonic-speaking peoples. Loegrish speakers from southeastern and East Midlands moved to the Scottish Lowlands, and set up a town called "Brithonis Nád", or "New Britain". Today, they claim descent from the ancient Picts, but this is, of course, not true.
The language is also spoken in Cape Newland in Canada and in the American northeast. They are descended from Loegrish people from Wessex, and claim descent from Celts rumoured to have travelled to "the land across the sea" (Tir iánc) with Saint Brendan.