Does anybody know the cheapest place where I can post, in bulk (60kg) from Thailand from the UK. It needs to take a month maximum but I would pay more for less time.

Dhl

All in one package? Regular postal service may or may not make it within a month, my experience is smaller/lighter packages travel faster.

Any other alternative is going to be expensive.

TNT is one of the cheaper airfreight options, for 60 kgs your probably looking at around 20k thb for a door to door service which should take about a week. Ups is generally little bit more expensive although only a few hundred baht but seems to take around 3 working days.