Name this movie

Love story set in England in 1930s. Guy writes a stupid, vile note to his posh upper class lover (I think he's just a menial) as a joke (that he laughs at and then dismisses, or so he thinks). He writes another, more serious and lovey-dovey note (this is after they have had serious sex in the drawing room or somewhere in the mansion) and gives it to her little sister to give to his love. Then he immediately realizes he's given her the "joke" letter and runs after her, but too late. Then the little brat accuses him also of raping his girlfriend's sister when she knew it was another guy. The police believe the little brat and he is thrown in jail. The war comes, and his ticket out of jail is to join the front lines. He gets wounded and dies, and his love dies herself when an underground shelter which she takes cover in is flooded. Yeah, it's pretty sad, but well-written and well-acted. I know somebody, probably many, people on here have seen it and even remember the name, I hope. Thanks in advance!