I flew to KL with Emirates then got a (sort of) connecting flight to f---it with Thai Airways. I think it was cheaper than the Jetstar (or is it Virgin) direct flight. The only downside is a 3-4 hour wait at the airport in KL.
Also be aware that the bus from Phuket Airport to Ao Nang is about 4 hours. Realistically it will take 5-6 hours to get from the airport to Ton Sai/Railay. So if you are getting in late you may have to layover on the island. As Wendy says, Krabi is closer so fly there if you can.