I'm not in the UK, but no one in my family has every used mommy or mummy. Growing up we called my mother mom, mum or mother, and my son calls me mom or mother, and his dad is just dad. I don't know why, but adding that -y to the end never felt right. Mum is a luvley word in either place though.
I like the variety of names you can choose from at the grandma stage though with a 17 yr old, I am in NO hurry to go there. My mom is Nana to my son but he also has a grandma, and an 'old grandma Ellen'.