This 35mm screening is part of the Re-releases strand at BFI Southbank (see here for full details) and this particular film is on from December 16th to the 20th.
Chicago Reader review:
There are no art deco nightclubs, shimmering silk gowns, or slamming bedroom doors to be seen, but this 1940 film is one of Ernst Lubitsch's finest and most enduring works, a romantic comedy of dazzling range that takes place almost entirely within the four walls of a leather-goods store in prewar Budapest. James Stewart is the earnest, slightly awkward young manager; Margaret Sullavan is the new sales clerk who gets on his nerves—and neither realizes that they are partners in a passionate romance being carried out through the mails. Interwoven with subplots centered on the other members of the shop's little family, the romance proceeds through Lubitsch's brilliant deployment of point of view, allowing the audience to enter the perceptions of each individual character at exactly the right moment to develop maximum sympathy and suspense. With Frank Morgan, Joseph Schildkraut, Sara Haden, and Felix Bressart.
Here (and above) is the trailer.