Halliwell was born to Laurence Francis Halliwell, who was of English/Swedish descent, and his wife Ana María (née Hidalgo), who was of Spanish descent from Huesca, Spain and who grew up in Watford, Hertfordshire. For a brief time, Halliwell's mother tried to bring her up as a Jehovah's Witness. Halliwell took her A-Levels at Camden School for Girls having taken her GCSEs at Watford Girls Grammar School.