Theres a few British files from WW2 that are still sealed, without getting into conjecture it's obviously pretty difficult to say why.

But as a few examples, there is the abdication of King Edward in 1936 and box 24, which was deemed still requiring sealing till 2037 when the rest of the boxes were opened (possibly as it relates to the Kings contacts with Hitler). Theres also the sinking of the RMS Lancastria when it was evacuating personnel in 1940, which was closed for 100 years right from the get go, and similarly the sinking of the Nazi prison ships in 1945 - one would assume the documents there show british intel knew more than they let on.

The biggest closed ww2 case in Britain remains of course, Rudolf Hess. When he landed in Britain, just how much did Hitler know about it, and just how much did British intelligence and society figures know about it? We'll have to wait till 2041 to find out....