Seen as this is the 1930s

I watched ITV's Harry Price: Ghost Hunter over the Christmas break, which starred Rafe Spall as the eponymous psychic investigator/faker/unmasker.

One linguistic fact of note is the spelling of his first name, which I assume is a respelling reflecting the pronunciation of the name Ralph, which is now usually pronounced as it looks.

But what caught my ear was the very modern-sounding language. One character used vowels which I would have said were characteristic of Estuary English or MLE: monophthongs where RP would have had a diphthong, for instance. And Rafe Spall himself said, at one point, seen as rather than the normatively correct seeing as (meaning more or less because or since). I'm hesitant to say that this is anachronistic, as it's almost invariably a 'recency illusion' when anyone makes an argument like that. Rather, it's probably just that we expect posh and middle class people in period dramas to speak in RP, and when they don't we find it jarring. But I can't find out how old this variant is, because it's virtually ungoogleable and I'm not sure where else to go to look for it. Seen as has indeed increased massively since 1960, but I can't filter out the false hits like we were seen as inferior, where it's not the same thing. Anyone happen to know about this? (Sorry if the chart is not the right size - html skillz are failing me today. Follow the link above to the original.)