The Labour government with Barbara Castle as its Secretary of State for health accepted the Butler proposals and ring fenced capital funding for the proposed regional medium security units. So there was a flurry of activity in 10 of the 14 regional health authorities in England (not Scotland or Northern Ireland) to convert existing psychiatric hospital wards into secure units, interim secure units (ISUs,) before the funds and plans were available to build purpose-built units.
The first ISU to open was at Rainhill hospital in Mersey in 1976, quickly followed by one in Prestwich and another at Knowle Hospital. A unit at Bethlem Royal Hospital opened in 1980 and those four served as role models for the rest of the country.
However the purpose-built units were long delayed. Even so by 1990 England and Wales had 19 functioning secure units providing 655 medium secure beds.
There has been quite a lot of expansion since that time and many of the original units have been enlarged or completely re-built as the parent mental hospital site which originally housed them was sold for other purposes.