Même texte en français ici. I'll copypaste the English version here in case of paywall. ... read full post
Small bit of info: Charles III still speaks RP, but the prince William (heir to the throne) already shifted to SSBE. Geoffrey Lindsey has a rather good video on that.
This is a rather long study, from the Oxford Studies in Ancient Documents. Its general content should be clear by the title, and it focuses on three "chunks" of the former Roman empire: Maghreb and Iberia, Gallia and Germania, and the British Isles.
Further info: the linguist in question is Lynn S. Eekhof, and she has quite a few publications about the topic, worth IMO reading.