IHTM11196 | Gifts to political parties: requirements for a qualifying political party
From HM Revenue & Customs · Inheritance Tax Manual
A political party qualifies for exemption if at the last general election preceding the transfer either
two members of the party were elected to the House of Commons, or
one member of the party was so elected and not less than 150,000 votes were given to candidates who were members of that party
These alternative qualifications are the minimum requirements.