Girl Guiding is a self described single sex organisation, and always has been. Let's just be clear here. In allowing boys who identify as girls enter Girl Guiding, GGUK's policy does break the law, under the Equality Act, but it's because they are discriminating against certain boys.
There is no such thing as a "transgirl" (i.e., an under 18) in law. Nobody without a Gender Recognition Certificate (GRC) is entitled to access single sex spaces (enshrined under the Equality Act). Nobody under the age of 18 can hold a GRC. A boy under the age of 18 who identifies as a girl is exactly the same legally as a boy who identifies as a boy.
Therefore, in allowing some boys (biological boys whose gender identity is girl) to come into girl guiding, but not other boys (biological boys whose gender identity is boy), they are directly discriminating against the latter. This is a clear breach of the Equality Act.
GGUK has been applying "Stonewall Law" which is their wrongful interpretation of the law.
This leaves us with massive issues in terms of safeguarding and indirect discrimination against girls from certain backgrounds (e.g., any girl that comes from a religion that does not allow her to mix with male bodied persons outside of her family unit). The dignity of girls and their boundaries has been thrown out the window, and gas lighting (telling them to believe someone is a girl, when all their senses tell them otherwise) and "kindness" over and above their own well being and safety has been imposed. Guide Leaders must treat a "transgirl" the same as a biological girl, so they must be allowed to share dorms, tents, toilets, showers etc. Parents of biological girls and the girls themselves are not afforded informed consent in this regard. Without your knowledge or consent, your 10-14 year old Guide may be sharing a tent with a biological (and biologically intact) boy of the same age, and all the hormonal urges that boy may be experiencing. This is safeguarding 101. Conversely, a female Guide leader who needs to take her own biological son on an overnight, because of no alternative childcare, is not allowed to share a tent with her own son.
As PP have said, Scouts is a completely separate organisation with a separate governing body. They have not been single sex for years, as a result of falling uptake, but also as a result of lack of male volunteer staff. Their overnight policies and safeguarding is much more transparent (no pun intended), because they are not labouring under a "fiction" that they are a single sex entity.