Who can initiate an optional referendum at the national level?

POLITICALPARTIES, ELECTIONS AND REFERENDUMS ACT

(Adopted in 2000)

http://www.legislation.gov.uk/ukpga/2000/41/contents

Article 104 Referendum questions.

(1) Subsection (2) applies where a Bill isintroduced into Parliament which—

(a) provides for the holding of a poll thatwould be a referendum to which this Part applies, and

(b) specifies the wording of the referendumquestion.