According to the NRE website, various TOCs are able to sell the TfL One Day Bus & Tram Pass and not just LUL and London Overground. Is this done through the ticket office or can you buy through TVMs as well?
https://www.nationalrail.co.uk/times_fares/95054.aspx
https://www.nationalrail.co.uk/times_fares/95054.aspx