The African Queen is definitely worth rewatching many times, and is a rare example of a movie that's better than the book. And I really liked Calendar Girls--two favorite stars, Helen Mirren and Ciaran Hinds. Movies I liked well enough to buy that also would work for group watching: Persuasion, the BBC version with Amanda Root and Ciaran Hinds is the best ever filming of this book--it does a remarkable job of portraying a story that takes place so much inside people's heads.
The BBC did a series of the Dorothy L. Sayers mysteries with Ian Carmichael as Lord Peter Wimsey. The best is The Nine Taylors, also her best mystery story. The background of a Norfolk village, bell ringing in a small parish church, the dramatic things that happen when the sluice gates break and the dykes flood, what it's like to be shoved into battle in WWI when you aren't a soldier, etc., are beautifully pictured. Unfortunately for your purposes, it's 3 1/2 hours long, so you either have to split it or have a very dedicated bunch of watchers. Five Red Herrings is also good, but is long too.