I'll be honest, I liked Cyril's development over the course of the movie. He was self-centred, pompus, rude-mannered and didn't care to help out in the beginning. But as the movie went on, we started to see snippets of their own homelife issues, such as their parents barely having time for them anymore, and I honestly felt sorry for Celia when their chaffeur returned in order to pass on the shoes.  

Celia started out as materialistic, uptight and spoiled. But learned to value hardwork and even went out of her way to help catch the piglets. The development of her relationship with Megsy was handled well, even if the two didn't have much screentime when compared to the boys and Uncle Phil.  

By the end, Cyril and Celia have changed into the best versions of themselves. Celia was vital in deactivating the bomb thanks to her sewing scissors and Cyril helped to uncover the truth in the War Office at London.  

His explosive outburst at his father was deserved too. The man was willing to talk dirty about his sister's husband, was rightfully bitten apart, and with a change of heart, revealed that he was missing in action, not killed.  Cyril and Norman were the MVPs of this film though. Love their relationship as bitter relatives becoming closer and learning to appreciate each other overtime.