The comprehensive agreement (SI1968/1100) came into force on 12 July 1968 and has effect
a) In the United Kingdom for
i) Income Tax from 1966-67,
ii) Capital Gains Tax from 1966-67,
iii) Corporation Tax from 1 April 1966.
b) In Luxembourg for periods of assessment beginning after 31 December 1965.
See however DT12259 as regards certain Luxembourg holding companies.
This agreement was modified by amending protocols (SI 1980/567) and (SI 1984/364) which had effect in 1975 and 1984 respectively. A more recent amending protocol (SI 2010/237) comes into effect in 2011 and brings in a new Article 28, Exchange of information. If using the HMRC Intranet, the protocol can be viewed through the “New treaties/protocols in force” link on the sidebar. On the HMRC web-site, searching for “Treaties in force Luxembourg” will provide a link to the protocol.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/luxembourg-tax-treaties