The reality is that much of what we know of the conversion of England in the sixth century comes from Bede. But the trouble is that Bede doesn’t seem overly well informed about sixth century England. In fact, it looks like until he got access to Pope Gregory’s letters, he didn’t even know when St. Augustine was sent to England to convert the population (sorry about the spoilers there). So it seems to me that so he probably doesn’t know the whole truth of the state of Christianity in England.
We’re told that by the time that the Roman Catholic Church returned to Britain, and St. Augustine landed in Kent, we’re told that he had to go far to the West to find Christians to speak with. But I don’t think that’s entirely accurate and there were probably Christians to be found if he looked in the right places.
At Iona, we know of at least two Christian Englishmen (Pilu and Generus) who were practicing at St. Columba’s monastery in what would become Scotland before the time of St. Augustine.
And then you have the example of the Roman villa in Lullingstone had a house church, which demonstrated that for a while there were some wealthy Christians there.
Then you have Bede’s claims that the Christian cult of St. Alban was maintained without break from Roman times. And major scholars such as Levison and Campbell accept his assertion.
And actually, there was another Christian cult…. the cult of Sixtus… that I find absolutely fascinating. The followers of Sixtus claimed he was a saint (in reality, it looks like he was a Bishop) but they couldn’t recall any specific facts of him. They didn’t know how he died, they didn’t know what miracles he had performed, and they were even a bit shaky on Christian practice.