Ancestry has just released a new database British Postal Service Appointment Books, 1737-1969 which covers people who worked for the telephone service as well
http://search.ancestry.co.uk/search/db.aspx?dbid=1933

Post Office: Staff nomination and appointment, 1831- 1969. Microfilm, POST 58, 80 rolls. British Postal Museum and Archive. London, England.
These British Post Office appointment books are indexes to the Postmaster General’s minute books. They show the point when a person began working for the Post Office or started at a new position within the Post Office. The books were kept from 1831 until 1969, when they stopped being kept because of legislation and human resource procedures.
Some records contain the job the worker was appointed to and the British Postal Museum’s website has the abbreviations in their Family History Guide. The index contains:
- Name
- Date of appointment
- Location

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