It feels like it should be another three episodes at least. Loose ends with how it looked as though the PE teacher would groom Ryan etc.
I think the ending needs Ryan definitively turning away from his dad, whether turning him in, saving Catherine from him etc. So far it has been Catherine saving him while simultaneously worrying how much of his father is within him / seeing what happened to her daughter. We see her being breathtakingly compassionate to people and situations that you might think she’d be hardened too - double de-clutch lady after she shot her son. The murderous CID cop etc but seemingly finding it hard in this series to keep extending that to the people close to her, Ryan, Claire, Ann etc - it seems that she’s not just retiring from the job but in some ways from her family, saving herself.
So I think Ryan needs to save himself from his father, and perhaps Catherine too for there to be a truly satisfying ending. I hope there is, it was so sad when he sad I love you to Catherine and she didn’t say it back. A million British mother’s hearts simultaneously broke!