MawBroon. I have not heard Marsh Mallow mispronounced but it might annoy me if I heard someone deliberately enunciating it wrongly. But said quickly, it is difficult to hear the vowel precisely. Also the sweet, surely, is named after the plant? The flowers of the plant are edible too.
I am terribly irritated by deliberate mispronunciations and wrongly used grammar. We've discussed it a lot here and that has given me some relief of my distress!
I certainly do not expect everyone to be correct all the time! Also regional variations are very interesting and full of character.
MissAdventure. I think both versions of migraine are acceptable. I was treated by Prof. Blau who authored the massive tome, "Migraine" and he pronounced it My-graine. It may, I think, have originated from the French which is pron. "Mee-gren" hence our use of the "me" rather than "my" at the beginning.
We had a great discussion about scones here not long ago, do you remember?
About "sounding posh". This has always puzzled me. I was born in a southern county in Britain. I studied in a northern university at 18. I was called a "snob" by so many people just because of the way I spoke! It was 1968. It upset me very much. I was by no means a snob and had no reason to be.