My impression (which could be way off) was that UKIP had been a major force behind the BREXIT vote.
The two main campaigns/groups were
- Vote Leave
(MPs and peers from the Conservatives, Labour, UKIP and other parties), backed by Conservatives for Britain
, the Labour Leave campaign
, and Business for Britain
- Grassroots Out/Leave.EU
(founded by UKIP donor Arron Banks, backed by Nigel Farage, former Respect MP George Galloway and DUP MP Sammy Wilson).
was designated by the Electoral Commission on 13 April 2016 as the official referendum campaign in favour of leaving the EU.