Wonderful movie! Even if you don't like cats or London or stories about drug addiction, you get carried along by it.
The pace is good, the scenery is great and the story is inspirational.
I also think the message might just be: responsibility takes you forward in life. James needed a reason to get clean and stay clean, and that reason was a cat that needed his care.
In South Africa there was a project some years ago to give hardened prisoners small birds to raise, and it was a great success.