While there are lots of cheapish options sending household belongings (not furniture) from Uk- Bkk there seems little the other way. Firms like JVK are about three times as expensive as sending back stuff that it cost to send out. Are there any alternatives to expensive companies like this door to door? If needs be I will have to go with somebody like this, but would like to find out if there are any reliable and cheaper options.

Apparently not?

I'm guessing that Thailand's trade balance with the UK and Europe is imbalanced so there's a lot more empty capacity in the return direction, hence the lower prices.

Or specifically in the household contents business lots of stuff gets shipped in with expats taking on contracts here, but many of them end up staying, or at least stay long enough that by the time they head home their stuff's worn out and isn't worth as much to ship back.