[mhh__]

I have never ever heard anyone call them that in the UK, they're either brackets or parenthesis to anyone I've ever spoken to - from scientists, to builders, my grandparents all said brackets (Including the one is almost illiterate)

I'm sure someone must use it but it's not at all common

[mst]

I picked up "round brackets", "square brackets", "curly brackets" somewhere during my (Lancashire) youth though generally I only use them now when I encounter somebody confused by parens/brackets/braces as terminology.

[mhh__]

I say square and curly, but not round - I'm from Devon, but I'm only 19, so I haven't got the accent.