I was about 5 years old when this film was made, but the backdrop (ruined builings, unexploded bomds, et al) is really accurate.
I recall that my father used to park his car on a bomb site where ther had been a butcher's shop, complete with a tile floor, and "Eastmans" threaded into the tile.
I frst saw this movie when I was about sixteen, and beginning to learn how government works. There is a tremendous credibility to the behaviour of the British Government, which hasn't changed too much.
Similarly, the resentment of all things "official" still exists in the United Kindom, thereby lending a plausibility to the plot.
I do have to add a small disclaimer - around 1965 I worked very briefly as mate on J. Arthur Rank's yacht.