Join the British Film Institute and hope that the films you want to see are programmed at the National Film Theatre or your local BFI-sponsored theatre. Some other repertory cinemas also occasionally show this stuff. Most UK films of any merit have probably now made it to VHS or DVD so try searching on-line in the usual places. However... I also like some of this sort of material and have been steadily recording off TV to watch on some future rainy day; but the "supply" dried up about 5-10 years ago.