Look at going directly to the airline, you are likely to remove the middle man and get the best prices.
Just go to: http://www.malaysiaairlines.com
and check out flight costs all the way to London for when you intend to fly to Bangkok.
Then call them up on 13 26 27 and get them to quote you a price for a flight that has a few months stopover.
The only thing you will have to be aware of is that you cannot reserve seats on any flights more than 11 months in advance almost 8 years ago
There are lots of options to fly through Bangkok. The stopover should be free as the plane is forced to stop to refuel in Asia before heading to Europe. Nearly all stop in Singapore or Bangkok. I am aware that Qantas, BA and Emirates do but there are lots of other options too. I did that once on the way home. There is lots to do in Thailand - an amazing place with super-friendly people. almost 8 years ago
Yeah I did the same thing, no problems or extra costs and I think its great to visit places along the way.
I got a mate though that swears by Air Asia. He's managed to score a flight from Melbourne to KL for like $150 or something ridiculous like that. Might be able to score something cheap that way. I think if you do your research you could probably ride the cheapo airlines all the way.
Air Berlin were offering flights from Beijing to Berlin for heaps cheap. Theoretically you could budget airline it (ie Air Asia or Jetstar) to Bangkok, then to Beijing then Air Berlin it to Berlin, then Ryanair or Easyjet it to London. Pain in the arse I know but if your anything like me and am a cheap bastard that likes to see lots of towns on the way then it might be worth a look. almost 8 years ago
If your looking at a cheap option go Air Asia. Most of their flights go through Kuala Lumpur but Bangkok is only a short flight from there. They have jusrecently started flying from KL to London. Check out their website www.airasia.com
you can get return London from KL for about $143 AUD right now. Unreal!
The seats aren't the confiest but if you get a spare next to you lay down and sleep like I did, it was fine. almost 8 years ago