Double Taxation Relief Manual: Guidance by country: Portugal: Agreement
The comprehensive agreement (SI 1969/599) has effect
In the United Kingdom for
i) income tax and capital gains tax from 6 April 1970,
ii) corporation tax from 1 April 1970.
In Portugal from 1 January 1970.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/portugal-tax-treaties