There are several large, multinational insurers that offer health insurance to expats.These insurers will have strong financial strength ratings from AM Best or Standard & Poor, which means that a policyholder does not need to worry too much about the insurers becoming insolvent. Nothing is guaranteed, of course, and AIG, once one of the world's largest insurers, almost bit the dust back in 2007/2008 despite excellent financial strength ratings just prior.
Cigna, AXA, Allianz, and insurers at Lloyds, among others, carry good financial strength ratings and if you take out a policy with them from one of their offices in a well-regulated jurisdiction and through a reputable insurance broker, you will at least have a proper regulatory environment and reasonable consumer protection to go along with financial strength.
My suggestion would be to find a good insurance broker outside of Thailand who deals with the Europe-based operations of major insurers. Discuss your needs with the broker and let them provide you with indications or quotes from major insurers. Others may be able to suggest an offshore broker.