In the book, Mrs. Pevensie never had a first name.
In the movie, they used Georgie's mom's name, Helen.
It's a nice touch because not only is it Georgie's name, but it is also the first Queen of Narnia's name.
It wasn't CS Lewis who gave the name, but it was very in keeping with Narnia, and a nice tribute.
It seems to me that in Narnia, if you were born there, nobody had first names and that those who were born in England did.
There are different traditions in Narnia, and perhaps the whole last name thing was apart of them.
If you notice, while in Narnia the four Pevensies aren't referred to by their last names by anybody there...they are King Peter, Queen Susan, King Edmund and Queen Lucy.
Interestingly, King Frank wasn't referred to as King Frank until he became the first King of Narnia. He was simply called the Cabby. Also, Frank seems to refer to his wife as "Nellie" though perhaps that could be a nickname from Helen.
This is the first reference to Queen Helen:
"If you please sir," said Queen Helen (for that is what Nellie the cabman's wife now was)...
It seems to be in tradition of Narnia.
Tumnus was only ever Tumnus the Faun.
The beavers were Mr. and Mrs. Beaver, but that is like being called Mr. Human and Mrs. Human - it wasn't their names.