Yes, I'm sure she does. "Mummy" - all the way into adolescence and even adulthood - is the upperclass choice. I think Prince Charles addressed the Queen as "Mummy" in a speech at some public event - 80th birthday perhaps. I've had a couple of (posh) friends who used the term - in conversation to others, not just in addressing said parent. "Mummy says I must come home for the summer." Have you read any of Ruby Ferguson's Jill books? She always talks about "Mummy" - never Mum - all the way up to seventeen I think.

My own (Welsh, working-class) mum made it very clear that "mummy" had a definite sell by date!