This might help date it to earlier than 1962.
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"The UK Met Office has used the Celsius scale – formerly known as “centigrade” – in its work since 1st January 1961, to allow for greater international co-operation and because of the convenience of the scale. As the Met Office reported in 1962, this “led to the consideration of the desirability of introducing it in weather reports and forecasts for the general public.”
With the agreement of industry and the government, the Celsius scale was given after Fahrenheit from January 1962 as an interim measure, and then from the 15th October, Celsius became the primary unit given, with Fahrenheit retained as a secondary unit to aid the transition “for a period of several years.”