This 70mm screening (also being shown on July 16th) 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.
Chicago Reader review:
David Lean finally got some madness going in his images with this 1970 production. In 1916 Ireland, Sarah Miles marries tweedy schoolteacher Robert Mitchum (his charisma hidden behind steel-rimmed spectacles), but soon begins an affair with Christopher Jones, a soldier of the occupying British army. It's insanely overproduced in Lean's standard epic style, yet somehow the crazy mismatches in scale contribute to the film's sense of romantic delirium. With John Mills, Trevor Howard, and Leo McKern; photographed by Freddie Young.
Here (and above) is the trailer.