Having taken Knutsford many times, I would suggest having a backup plan in case your bus to the airport does get cancelled.
Overall, Knutsford is a fabulous option...but I've had them late by a few hours & even completely cancelled until the next day on another trip. I had the ability to change my plans when this happened, but I've never had to nor would I rely on Knutsford to get me to the airport on time.