From the longer Wikipedia page [1] - which contains a list of companies whose records are held at the archives.

BT Archives is an archive preserving the documentary heritage of BT and its public sector predecessors. It is designated an official place of deposit for Public Records, for those records created prior to BT's privatisation in 1984.

The records include minutes, subject files, photographs, film and advertising material that tell the story of telecommunications in the UK and from the UK to overseas, from the formation of the private telegraph and telephone companies in the nineteenth century to the present day.

The website is [2].