In British English, the term "Nanny" can indeed be used to refer to a person who works as a child's caregiver or a nursemaid, similar to what is known as a "nanny" or "au pair" in other cultures. However, in many British families, "Nanny" is also used as an affectionate term for a grandmother.
The use of "Nanny" as a term for a grandmother is a form of endearment and is more common in certain regions or families. It's a tradition that has been passed down through generations in some families, and the choice of using "Nanny" or other terms like "Grandma," "Granny," "Nana," or "Grandmother" often depends on personal preference, family tradition, or regional dialects.
The British English language is diverse, and different families may use a variety of terms of endearment for their grandparents. Some families may even use different names for different grandparents (e.g., Nanny for the maternal grandmother and Grandpa for the paternal grandfather).
Ultimately, the choice of what to call a grandparent is a matter of family tradition, preference, and individual relationships. It's important to remember that language and family dynamics can vary widely from one family to another, and terms of endearment like "Nanny" for a grandmother are part of the rich tapestry of language and culture in the United Kingdom.