I'm sure a lot of us know that BBC wiped tapes in the 70's which of course did happen to Top Of The Pops, making it so they themselves have lost many performances.
But I know of a few backups, first being private collectors. Many private collectors have copies of lost episodes, and sometimes send them to the BBC where they could possibly reair on BBC4, although many also trade tapes. I knew of one collector who had some lost queen tapes but he ended up vanishing mid-trade. Another backup is regional variants, I don't think this was super common but it did happen. From what I can tell, episodes would be recreated in other countries with different hosts speaking different languages, but they would use the original performances from the BBC series. This was the case with King Crimson who had a long lost performance of the song Cat Food, that only survived thanks to a foreign edition (allbeit in black in white).