This 35mm screening (also being shown on July 22nd) is part of the Christopher Nolan Presents ... season at BFI Southbank, dedicated to showing movies from prints. You can find all the details about the season here.
Time Out review:
Despite the now rather embarrassing propagandistic finale, with McCrea urging an increase in the war effort against the Nazis, Hitchcock's espionage thriller is a thoroughly enjoyable affair, complete with some of his most memorable set pieces. Joel McCrea and Laraine Day are the lovers searching out Nazi agents in London and Holland after the disappearance of a peace-seeking diplomat, while George Sanders, Edmund Gwenn and the normally wooden Herbert Marshall lend fine support. Something of a predecessor of the picaresque chase thrillers like Saboteur and North by Northwest, its main source of suspense comes from the fact that little is what it seems to be: a camera hides an assassin's gun, sails of a windmill conceal a sinister secret, and the sanctuary of Westminster Cathedral provides an opportunity for murder. Not one of the director's greatest - there's little of his characteristic cruelty or moral pessimism - but still eminently watchable.
Here (and above) is the trailer.