KL40: Latest editions of Jowitt’s Dictionary of English Law or Stroud's Judicial Dictionary (both available online to holders of an Oxford SSO via Westlaw UK)
Charities Act 2011 c 25
s. 3 (2) In subsection (1)—
(a) in paragraph (c), “religion” includes—
(i) a religion which involves belief in more than one god, and
(ii) a religion which does not involve belief in a god
(Also s.4 stipulates public benefit requirement)
R (on the application of Hodkin) v Registrar General of Births, Deaths and Marriages  UKSC 77,  AC 610 Lord Toulson said ‘religion' could be described as ‘a spiritual or non-secular belief system, held by a group of adherents, which claims to explain mankind's place in the universe and relationship with the infinite, and to teach its adherents how they are to live their lives in conformity with the spiritual understanding associated with the belief system'.
Since 2001 Census returns have included the voluntary question asking them to declare their religion. As well tick box options (No religion, Christianity, Buddhist, Hindu, Jewish, Muslim, Sikh) there was "Any other religion, write in." The result was an extensive list, including some new entrants.
In 2015 a Win/Gallup International Association poll found that only 30% of people in the UK considered themselves religious.