Wow this is the first time I can post a response and actually have a source. In Chris Wickham's 'The Inheritance of Rome' he makes the case that Roman control over England was less ingrained and overall fell off about a century earlier than elsewhere in Western Europe during the decline. The Western Empire's reach slowly contracted over a long period, ~150 years, perhaps longer, and naturally England was more or less the first area lost due to distance from the heart of the empire which was the Mediterranean (I'm fuzzy on the dates or if the Vandals had already started breaking away North Africa at this time, I believe England was lost/given up earlier).

Furthermore, most other provinces underwent a gradual shift from Roman to a localized State of some sort. While these devolved in their own ways, they often, at least initially, kept some Roman and Christian structures. Latin being the primary language was one of these. In fact in the early stages of the transition most citizens of these splintered regions would have still called themselves Roman. It was a long process of ceeding power before actual new states really emerged. In essence the newcomers lived in a Roman structure as power was shifted.

On top of this, he points out that the germanic or frankish groups probably never reached over 10% of the population in their individual areas. In various ways they assimilated themselves to some of the norms predating them. In contrast, in England you then had the Anglo Saxons come in post Roman hegemony already having mostly departed. They brought with them their own culture and language and slowly started to expand their reach around the south east of the island, replacing the Britonic language that had been there.

I highly suggest you pick up the book if you are at all interested in Rome's transition into the medieval states. And apologies if I have any facts wrong. I'm a historian by hobby not by training, and often keeping the million names and dates can be tough for me. The above was roughly 400-550 ce. for the desolving of Rome and then 550-750 ce. for the emergence/divergence of distinct states.

And since I forgot to answer your first question... my stab at it is that proper latin really lived on with the aristocracy. Britain of the time had the weakest least wealthy aristocracy of the post Roman west. This alone to me would point to latin being less common. I forget exactly how widespread Wickham says it was but as I recall Britonic languages and later Anglo ones were much more in use than latin. I also believe the Romans didn't settle in England in the same way as other areas. They didn't adapt British culture to theirs as aggressively. It was much more literally a series of outposts and small cities.

While latin would have also been the language of Christianity, this too was less developed as an institution in England as it was in continental Western Europe.

Also citing Chris Wickham's Inheritance of Rome here.