Brook was born Kelly Ann Parsons in Rochester, Kent, the daughter of Sandra Kelly, a cook, and Kenneth Parsons, a scaffolder. She has a younger brother, Damian, and an older half-sister, Sasha. Kenneth Parsons died, aged 57, in Rochester from lung cancer, on 26 November 2007, during Brook's time on Strictly Come Dancing. Brook attended the Thomas Aveling School in Warren Wood, Rochester. In 1996 at the age of seventeen she appeared in the second series of "Fist of Fun", the BBC comedy show from Stewart Lee and Richard Herring as a schoolgirl in a series of sketches entitled 'Teachers' (one of her co-stars was the young Daniel Mays). She studied at the Italia Conti Academy of Theatre Arts in London for three years during this time before becoming a professional model at the age of 16.
In February 2007, it was announced that Brook had signed a contract, reported to be worth around £1m, to represent Unilever's Lynx body spray, known as "Axe" in the US and in continental Europe. She has appeared on billboards, in newspapers and on-line as part of their advertising campaign. She has also appeared in commercials for Sky+ and T-Mobile and modelled for Reebok.
In 2007, Brook participated in the celebrity dancing competition Strictly Come Dancing on BBC1, with dance partner was Brendan Cole. During the TV series, her father Kenneth Parsons died from cancer, and although she initially decided to continue dancing in his memory, she withdrew from the competition in week nine.