When was the UK TV watershed created?

In 1958, the Nuffield Foundation funded social psychologist Professor Hilde Himmelweit at the London School of Economics to look at the influence of television on children in Great Britain - prompted by the increased popularity of television since the introduction of commercial broadcasting in 1955. The end result of this was the publication of "Television and the Child".
Professor Himmelweite's research - which questioned over 4000 children - found that, although parents couldn't be held responsible for what a child might see on television, few children watched TV after 9 pm. The solution she suggested was that "unsuitable" material be excluded from programmes before 9 pm.

In 1959, the Independent Television Authority (ITA) and the BBC jointly appointed a committee chaired by Ms May O'Conor to study the recommendations. Both the BBC and ITA said they did not believe that the needs of children could be allowed to determine the nature of all TV output up to 9 pm but conceded that 6-9 pm should be seen as "family viewing time".

Officially, the watershed was introduced following the Television Act of 1964 (a consolidation of the Television Acts of 1954 & 1963, which set out the rules under which Independent Television companies were allowed to operate). Its stated aim was to exclude all material that might be injurious to children, before 9 p.m.

Technically, the BBC weren't covered by this requirement but operated under its own charter, instead. It wasn't until the 1980's (confirmation required) that the BBC announced their commitment to a formal 9 p.m. watershed.