Does anyone know of any companies that will ship a 30 kg from Samui to the UK and a rough estimate of the costs involved?

try the post office or fed express in chaweng or they is one in tesco lamai not sure on cost though.i shipped 5 phones out to the uk from the post office and it only took a week.

Via Thai Post depending on service between 6000 and 12000 Baht. Couriers like DHL / FedEx are generally around 80% - 200% more expensive. But there are limits on size. If it is too big, you'l have to find a freight forwarder who can send it LCL (less than container load) over sea. That shouldn't cost more than 5000 baht. (Over sea there are restrictions on using wood packaging)

when i sent something from bali it was cheapper to use a http://www.parcel-link.co.uk/?gclid=CJW6oZ-PwK0CFUIa6wodOi_LAw DHL in bali wanted £245 and only paid £190 for 30kg though here might be different?