The Key Man (1957) is a fine British crime B-picture with a definite hint of film noir.
You Can’t Escape (1956) is another low-key British suspense thriller B-movie that is well worth a look.
Christmas Holiday (1944) is a second-tier American film noir directed by the great Robert Siodmak. Worth seeing for Gene Kelly as a creepy and totally untrustworthy pretty boy anti-hero. It's also musical star Deanna Durbin's only film noir!
The Mirror Crack'd (1980) is a fun adaptation of the Agatha Christie with Angela Lansbury making a rather good Miss Marple. Plus it has delightfully high camp performances by Elizabeth Taylor, Rock Hudson, Kim Novak and Tony Curtis.
The House of Fear (1945) is one of the lesser Basil Rathbone Sherlock Holmes movies but it has its moments.
Circle of Danger (1951) is a interesting Jacques Tourneur-directed British mystery thriller.
Guilt Is My Shadow (1950) is an understated but effective little British crime thriller.