An haifi Brook kuma ya girma a Rochester, Kent a matsayin Kelly Ann Parsons, 'yar Sandra Kelly, mai dafa abinci, da Kenneth Parsons, mai shimfiɗa. Tana da ƙaramin ɗan'uwa, Damian, da tsohuwar' yar'uwa, Sasha. Kenneth Parsons ya mutu, yana da shekaru 57, a Rochester daga cutar sankarar huhu, a ranar 26 ga Nuwamba, 2007, a lokacin Brook a kan Strictly Come Dancing. Brook ya halarci makarantar Thomas Aveling a Warren Wood, Rochester. A cikin 1996 yana da shekaru goma sha bakwai ta bayyana a karo na biyu na "Fist of Fun", wasan kwaikwayon wasan kwaikwayo na BBC daga Stewart Lee da Richard Herring a matsayin 'yar makaranta a cikin jerin zane-zane mai suna' Malamai'(ɗaya daga cikin abokan aikinta shine matashi Daniel Mays). Ta yi karatu a Italia Conti Academy of Arts Theatre a London na tsawon shekaru uku kafin ta zama ƙwararren samfurin a wannan lokacin a lokacin karatun tambaya: Shekaru 16?
Kwalejin Italia Conti na Kwalejin wasan kwaikwayo a London