The Saturdays singer has become friends with the British royal through their England rugby player partners, Ben Foden and Mike Tindall, and she has revealed Zara - who is Queen Elizabeth's granddaughter - is great to hang out with.
However, pregnant Una admits she was disappointed not to be able to have any alcohol at Zara and Mike's wedding in July.
Una - who is expecting her first child with Ben - said: "Zara is brilliant, so much fun. I was disappointed I couldn't have a few drinks and have fun with her, because she knows that's what I'd normally be like.
"It was such a beautiful wedding - the setting, ceremony and weather - everything was perfect."
At the time of Zara and Mike's wedding, Una had not announced her pregnancy so claimed she had been stricken with tonsillitis in order to avoid questions about why she wasn't drinking alcohol.
She said: "I was quite nervous because I knew there'd be a few drinks going down and it would be noticed I wasn't touching alcohol. I had to lie and say I had tonsillitis, which was clearly untrue as I was chatting away. I felt awful about that."