My father worked as a "sensitivity reviewer" for a UK government records office in his retirement after he left the diplomatic service. The stuff he was releasing was from mostly 50 years years ago when it was statutorily made public as long as it didn't deal with anyone alive (or members of the royal family - thanks Tony Blair!). He said he'd heard that there was a "black hole" coming down the line from the period in 90's when departments moved to email and digital storage. People in about 2010 trying to get stuff off early archives of those for internal purposes and FOI requests were reporting they were often unable to do so.
Maybe that's a good thing, I dunno. But for anyone wanting to know what was being said about, for example, Pergau Dam in internal communications between Whitehall and the Foreign Office - they might have a hard time.