According to the BBC Year Books, which are all available online, Windermere's service date for UHF TV was 1971. VHF was in service in 1971. In 1970, no service of any sort emanated from Windermere.
So I would say 1971 is the date you are looking for. However... The 1972 volume claims that year as a service date, so perhaps the project over-ran?
https://www.americanradiohistory.com...k_Page_Key.htm