I didn't say it had anything to do with 'diction, spelling, grammar, etc.' I said the opposite, in fact. RP is not 'basically unused'. It is the standard accent taught to non-native English speakers. Hell, it's even called 'BBC English' (cf. 'Walter Cronkite English' in America as an offshoot). No, BBC English is not RP. They're derived from different accents. RP is is more Kentish, while BBC English -- the more modern of the two -- is derived from the "University English" of the area surrounding London (Oxford, Cambridge, etc.) As for diction, spelling and grammar, I'll repeat: you equated Commonwealth English -- which IS mostly about diction and spelling -- and RP, and they are not the same thing. The Canadian example you quote is still a dipthong; it is pronounced by most Anglo Canadians as /Λu/, roughly. But the previous words you were talking about were never dipthongs in English, only French. The diagraphs were used simply to show French origin (as opposed to words from Latin, like governor, which have rarely been spelled '-our'), not English pronunciation.