According to the new EHRC code, the Girl Guides can go back to being girls plus tran-identifying boys. See para. 12.74 and 12.75, and example 12.77.
12.74 Section 6(c) of the Interpretation Act 1978 states that, in any act, words in the singular include the plural and words in the plural include the singular, unless the contrary intention appears. Applying this provision, the Equality Act 2010 (the Act) permits associations to be based on more than one protected characteristic.
12.75 This interpretation of the Act is consistent with the right to freedom of association under Article 11 of the European Convention on Human Rights (ECHR). Article 11 includes the right (subject to proportionate restrictions) for people to be able to associate with one another to the exclusion of others in appropriate circumstances.
An association may therefore restrict membership to people who:
share one protected characteristic (for example women)
share more than one protected characteristic (for example Muslim women)
have any of more than one protected characteristic (for example lesbian, gay, bisexual or trans people)
Example
12.77 A lesbian, gay, bisexual and trans (LGBT) support group is set up to include gay, bisexual and trans people. Its membership is restricted to people with the protected characteristic of any of those sexual orientations, or gender reassignment. This is permitted under the Act.