What is the most common term for grandmother and grandfather in the UK?

Grandma and Granddad is the most common.

But some other backgrounds include the names; Nan, Pops etc. etc.

We actually have quite a few terms for Grandfather and Grandmother they are;

Grandfather: Grandpa, Grandfather, Grandpop and the most common one used is Grandad.

Grandmother: Grandma, Granny, Nanny, Nannie and the most common one is Nan.

It depends, everyone i know, and all the cards in the card shops say Grandad. But Grandma and Nanny are equally used for a Grandmother depending on what the person likes to be called. I would use Grandma though so nobody get's confused with a babysitter. (Which can also be reffered to as a Nanny).

In Britain, Ireland, United States, Australia, New Zealand and Newfoundland and Labrador, Nan, Nana, Nanan, Nannan, Nanna, Nada, Nanny, Gran and Granny and other variations are often used for grandmother in both writing and speech.

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This is a portion of the referenced website. I hope it is helpful.

Grandma

Granny

Grandad

Gran and Granddad.

Gran and Granda with my family (we're from N. Ireland)

Grandmama and Grandpapa, depending upon which class one belongs to.