DH is from near Stourbridge, and although it is pronounced more like Mom, I've only seen his mother write it as Mum.
I'm not from round'ere, but it was always Mummy, then Mum when I was little. My sister and I went through a phase of saying Ma, but that was an affectation because we liked it from the Little House books.
My DC all call me Mummy (eldest is 13). I didn't know it made me posh!