Passport To Treason (1956) is only very slightly noirish but it's an enjoyable British spy/private eye movie that looks pretty good for a low-budget flick. And it does have lots of shadows, night scenes, fog and plenty of paranoia. Very much worth a look.
The Interrupted Journey (1949) is a British crime flick with some slight claims to being film noir and a rather intriguing premise involving trains and infidelity.There are some very clever ideas but it self-destructs in a disastrous way at the end.
The October Man is a 1947 British mystery/suspense film with just a slight film noir touch to it. It's quite stylish and features a fine performance by John Mills as a man with a mystery of mental trouble who finds himself suspected of murder.