The agreement (SI81/1546) has effect
a) In the United Kingdom for
i) Income Tax from 1981-82,
ii) Capital Gains Tax from1981-82,
iii) Corporation Tax from 1 April 1981,
iv) Petroleum Revenue Tax for any chargeable period beginning on or after 1 January 1981,
v) Development Land Tax for any realised development value accruing on or after 1 April 1981.
b) In Thailand for taxable years and accounting periods beginning on or after 1 January 1981.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/thailand-tax-treaties