Lynne is on the phone. She tells the person on the line that George has said that they need to economise, which Lynne has agreed to. The person on the line tells Lynne that that shows her which side her bread is buttered on. Lynne says that is neither right now so that they can save money.
After yesterday's shopping trip, Lynne liked 'Mantha's underwear so much that she bought herself the same set. Even though George has told her about their financial worries, once more she has spent money on clothes. Underwear is essential though, so maybe George will let this one go.
Whoever is on phone to Lynne is trying to diplomatic on which side to take. This person is probably friends with both George and Lynne so would not want to upset anyone. Lynne is told that her decision shows which side her bread is buttered on, meaning that Lynne is loyal and respectful to George. Had Lynne disagreed with George, the response would have been the same. This person doesn't care about Lynne's problems as they have their own problems. This person is probably 'Mantha.
What a coincidence that Lynne is actually eating bread at this time. Plain white bread. George's first rule of economising is always that spreads and condiments are surplus to requirements. These weekends away won't pay for themselves if they insist on frittering away their money on such luxuries as butter and ketchup. Out goes the marmite, jam and salad cream. If things get worse, sugar and eggs will be the next to go.