The asylum opened in 1851, and was the largest asylum in Europe at the time.
By the 1880s, public perception of Colney Hatch and its patients caused the words "Colney Hatch" to be used to refer to anything unusual or irrational. This is similar of how the Tanjung Rambutan in Malaysia, in which it's insane asylum, the Ulu Kinta Mental Hospital or Hospital Bahagia is situated, and is colloquially used to refer to insane asylums in general.
The Fight at the Lamp-post
Shortly after Jadis arrived on Earth, she caused a scene by stealing some jewelry and a horse. When she was stopped in the street, she loudly declared who she was, only to be ridiculed by the crowd: -
At first, Jadis was pleased and gave the crowd a bow. However, she soon realised that they were making fun of her. In response, she hit the chief policeman on the head with a bar that she had broken off from a lamp-post, and threatened to destroy the whole city. Digory and Polly, however, used their magic rings to take the witch back to the Wood Between the Worlds before any real damage was done.
- Information on the asylum from the Barnet London Burough website.