Fionn,

I feel that a more important reason for the maintenance of the thirty year rule on BP/Enigma was the existence of the British Typex (Type X) mechanical cypher machine, Typex was a developement of the enigma machine, was in use by both Britain and her "allies", and could be broken with equal facility by GCHQ.

SIS really wouldn't want our "friends" to know we were capable of (and were) reading their encyphered messages. (Assuming the sort of weak operator discipline that allowed Enigma to be broken was present.) When the story of BP was eventually released, Typex had gone out of use.

I don't think Churchill would have had a major problem justifying, and standing by, his decisions given his stances on other issues during his career.

AndyG