In over 40 years of working, and later living in retirement, in Thailand I have never been asked for any form of 'legalisation' of my UK passport/passports. However, I married my Thai wife in Taiwan and had the marriage certificate approved and stamped by the Thai embassy in Hong Kong, which was accepted in Thailand by the Thai immigration authority when applying for an annual extension based on marriage. The validity of my UK passport has never once been questioned during these procedures.