The 25-year-old singer - who releases her second album in October - met the heir to the British throne when she was 13 and she will never forget her royal encounter because he was such a lovely, done-to-earth man.
She said: ''When I was about 13, I was in a musical and he [Prince Charles] came to see the musical and I got to meet him afterwards and he was really cool.''
Eliza comes from a very successful musical family. Her father is John Caird, the stage director and playwright as well as an Honorary Associate Director of the Royal Shakespeare Company. Her mother is Tony Award-winning musical theatre star, Frances Ruffelle. Eliza is also the granddaughter of Sylvia Young, founder of the theatre school.
She recalled: ''We all had to live in this house and it was a bunch of kids in this musical thing. We were addicted to 'Tony Hawk's Pro Skater 2' and he was asking us what we did when we were in this house and we told him and he was like, 'yes I know that game! My sons play that game.' He then said, 'I like it, I gave it a go the other day.' It was the best thing - he was a hero.''
Prince Charles became a grandfather this week when William and his wife Duchess Catherine - formerly known as Kate Middleton - welcomed their son Prince George Alexander Louis into the world.
The '12 Years A Slave' director will receive the accolade at the London Film Festival in October.
Critics from all over the world were asked to name the best movie of the past 16 years.