She was born Hilary Mary Allcroft, in Worthing, England and, at 16, changed her name to Britt as her career in British radio and TV gained momentum. She went on to create a string of shows for the BBC and ITV networks, including "Moon Clue Game", "Get it, Got it, Good", "Dance Crazy", "Keepsakes" and "Mothers by Daughters" . She worked in live theatre staging shows at the London Palladium and Drury Lane Theatres.
In 1979, Allcroft was making a five-minute film for the Central Office of Information about the resurgence of interest in steam railways. One inspiration for the film was A Guide to the Steam Railways of Britain, a book compiled by the Rev. W. Awdry and Dr Christopher Cook, and published by Eric Marriott, the former editor of The Railway Series. As Awdry would be taking part in the programme, Allcroft was prompted to read some of the volumes of the Railway Series, for the first time. She said: "it really didn't take me long to become intrigued by the characters and the relationships between them and the simplicity of the stories. I was equally intrigued by the illustrations, the landscape in them and the nostalgia they invoked." She met Awdry, saying she wanted to make a TV series, based on his stories. Although Awdry was initially reluctant, a deal was eventually done. It took Britt three years to raise the finance and find all the people who could make her dream come true. The first episode of Thomas the Tank Engine and Friends, was aired in the UK in September 1984.
In 1984 she co-founded the Britt Allcroft Company with her then husband, Angus Wright, formulating cover of all merchandising and television rights in Thomas the Tank Engine and Friends. The company went public in 1996 and was renamed Gullane Entertainment.