This is really an interesting question...no sarcasm in that. I don't know how to answer but How could a person convey except thru video how it sounded back then? I would like to see if someone could do it by text..but hey thx for participating in AskHistorians with an original question besides all of mine.

Posh people from Wales, Ireland and Scotland all sound English even to this day

Having presumably been sent to public school from an early age, it will be some form of RP.