Any thoughts? Maybe she still lives there but has gone away - giving him the chance to plan the big job of the storyline, and his chance to be free of her?
Look closely at the portrait, and then go to THE LIFE AND DEATH OF COLONEL BLIMP. It's the same portrait of Deborah Kerr (they've muted the colors) that Clive Candy shows to Theo of his young, deceased wife. I think it was an inside joke because Roger Livesey was in the cast, and he, of course, was Colonel Blimp!