The captain of an aircraft orders the fuel for each flight based upon the calculated weight of the aircraft. Excess baggage means excess fuel, so that's why you have to pay. However, the amount of extra fuel required depends upon the distance being flown. For this reason, excess baggage charges for long haul flights are often considerably more than for short-haul travel.
Each airline will have it's own way of calculating excess baggage charges but when I flew, with Qantas, from Adelaide to Heathrow, they tried to charge me an excess baggage fee which was greater than the return fare from the UK to Australia. I managed to beg and plead and pay a more reasonable figure but you should be aware of the way they worked out the charge. (Other airlines might use a similar policy and not be so lenient in their interpretation of the rules):
Qantas calculated the charge as 1% of the price of a full-fare First Class ticket for the journey. I've just checked out the price, with Thai Air, of a similar ticket from London to Bangkok. The figure quoted was �5582.50. That means, that if Thai Air operate the same policy as Qantas, you could be asked to pay around �840 for your excess baggage .
The Thai Air website isn't particularly user-friendly. It takes quite a while to find the page which refers to excess baggage charges - and then it only says that 'these are available on request'. I strongly recommend phoning Thai Air to get a definitive answer to your question, rather than risk having to pay a massive bill: 020 7491 7953.
Chris