Great question — answering as the UphillTech team:
Until they can get around to digitising all the back catalogue, they're stored in their original format - film, video tape etc, in huge data storage warehouses. From BBC Archives BBC Archives has over 1.5 million items in the Television Collection. The earliest recordings are on film and date from the end of 1946. Greater coverage of Television output is kept from 1948 and covers multiple formats including 35mm and 16mm film, 1” Videotape, Betacam, Umatic, D3 as well as digital files. The Television Collection is stored within environmentally safe conditions in vaults where both temperature and humidity levels are monitored and regulated. BBC Archives is digitising many of its older tape...
Full answer by Tetsujin: https://movies.stackexchange.com/a/115745 (CC BY-SA 4.0).
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