Assume you are from UK so flying from London. Since you don't mention how long you need to travel for, people can't really help you. It's currenty about 505 pounds for cheapest RT to BKK for 60-90 , but perhaps , as you suggested,one way is better. BKK is the cheapest in general to fly in and out of, next would be KL, and Singapore last due to taxes etc. GIven the increase in ticket prices when you go one ways, probably best just to buy a RT to BKK and then get back to BKK for return flight via air asia, bus train etc. You could also buy air only to some place like Phuket using a package tour company . Over the years, there is usually very cheap tickets on Thai in June one way to London, but this is not guaranteed and you must buy last minute. Thai calls them Super SAver fares and this is only in JUNE during the very low season....Again, don't know how long you are going for..
Use this search engine to play around for flights. matrix.itasoftware.com