On accident is one of those American sayings that sounds weird to my English ears. Like poop. where did the second p come from??? That said, I think our dialects are merging, (well more likely British English is becoming Americanised, but you guys make all the tv, so that's to be expected), for example, when I was younger no one said "ass", it was always "arse", but nowadays it's nearly always "ass"; except for the o…
https://books.google.com/ngrams/graph?content=poo%2Cpoop&yea...