Thank you for these details. For this this small carton at 26 x 21 x 22 cms and 1.5 kilos .. cheapest method will be by courier service, at cost :
GBP£
Collection from Cambridge and transfer to Courier at LHR = 39.00
Documentation / processing = 25.00
Courier charge to delivered in Bangkok = 40.00 minimum
Alternatively, perhaps shipper has an account with a courier company, and they could arrange to send it themselves, which will probably work out cheaper overall.
I hope this information will be of some assistance