Use the guidance on Letters (in an archive or collection) to reference letters found in physical archives.
You will need:
Author Surname, Initials (Year) Letter information eg. sender and recipient, Day Month. [Letter]. Name of Archive. Catalogue/ reference number. Available from: www.webaddress.com [Accessed day month year].
Wallace, F.A. (1940) Frank Wallace to Winifred Wallace, 12 January. [Letter]. BBC WW2 People’s War. Article ID: A8793895. Available from: https://www.bbc.co.uk/history/ww2peopleswar/stories/95/a8793895.shtml [Accessed 5 March 2021].
In-text citation: (Wallace, 1940)