Funnily enough I actually start back there next month. ISTR back in the days before MAM's and sizeable disc arrays, everything for archiving went onto (or stayed on) the tape format du jour. Over the years this has been 2" Quad, 1" C-Format, Beta, DigiBeta, D3 etc. This caused massive headaches when suitable playback machines like the quads became scarce. I think at the end of the 90's there was a concerted effort to get all the older stuff onto DigiBeta.

Nowadays I believe the BBC's DMI (Digital Media Initiative) is looking to store everything in a file based format, so I assume the archive is now a metric fsckload of LTO tapes. Having been bitten in the arse by erasing important stuff in the past, I believe they keep everything. I presume after a decade or so, they probably review it and bin off the unimportant stuff. I think they're also ingesting all the important stuff off videotape too.