The comprehensive agreement (SI 1988/932) has effect
a) In the United Kingdom for
i) Income and Capital Gains Tax from 1989-90,
ii) Corporation Tax from 1 April 1989.
b) In Turkey, from the fiscal year beginning 1 January 1989.
If using the HMRC Intranet, the agreement can be viewed through the “New treaties/protocols in force” link on the sidebar. On the HMRC web-site, searching for “Treaties in force Turkey” will provide a link to the agreement.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/turkey-tax-treaties