Issues in relation to which referendums are optional at the national level

United Kingdom
POLITICALPARTIES, ELECTIONS AND REFERENDUMS ACT
(Adopted in 2000)
http://www.legislation.gov.uk/ukpga/2000/41/contents
Article 101 (2).
Inthis Part-
(a)'referendum' means a referendum or other poll held, in pursuance ofany provision made by or under an Act of Parliament, on one or more questionsspecified in or in accordance with any such provision;
Political Parties, Elections andReferendums Act of 2000 provides the legal framework under which nationaland/or regional referendums may be held. Nevertheless, parliament must pass aspecific law in each instance in order for a referendum to be held.