@Stuart ********
I'm glad I've seen this. I've never been asked by an airline to show proof of onward travel and I've always had it so would have been OK but I'm taking advantage of China's relaxed visa rules and visiting China on the way from UK then coming to Thailand on a one way ticket from China.
If I hadn't seen this post I might have come next time without an onward ticket, thinking I could book that after arrival but now I'll be sure to have it booked before my flight from China into Thailand