In short: none whatsoever.
The EHRC statutory guidance on schools, which schools have to abide by suggests alternative arrangements for a child who identifies as trans and does not wish to use the facilities provided for their own sex. The EHRC does not suggest putting such a child into opposite-sex facilities.
So, you could contact the EHRC and ask them for help in ensuring that your daughter's school upholds the rights of all children.
Furthermore, the school has to have parental consent for school trips, you must be able to give informed consent to your child taking part in the trip. But for informed consent, the information provided must be full and explicit. By withholding vital safeguarding information from you, the school actively prevented you from giving informed consent. You did not and would not have consented to your daughter sharing a dorm with a male pupil.
Moreover, the UK is a signatory to the UN Convention on the Rights of the Child. The school breached the rights of your daughter under several of its articles. Foremost of those are her right to privacy (Article 16) and her right to have a parent or legal guardian safeguard her rights and help her exercise them (Articles 5 and 18). The right to have the best interests of all children, including your daughter taken into account when making decisions affecting her (Article 3). The right to have her views taken into account (Article 12) - she wasn't even asked, was she?
And not to forget, especially at a time when sexual harassment and violence in schools are endemic, Article 39, which puts an obligation on the state, and state actors, to protect child survivors of abuse and to help them to recover. You don't do that by putting female children (who are predominantly the victims of abuse) into mixed-sex dorms and you don't do it if the only way such a child can avoid a mixed-sex dorm is by disclosing the abuse, because that is retraumatising the child rather than protecting her from harm.
I mean this completely undermines safeguarding measures as well as teaching girls how to keep safe. If we don't allow girls to assert boundaries around their own bodies against all male children, then we're not safeguarding them properly. Every time we create a sacred caste that is exempt from normal safeguarding rules, we end up with children and women suffering abuse.
And the way this is done is insidious. So be kind now requires girls to ignore their own discomfort and safety. Great.
It would have taken an incredibly confident and assertive 12-year-old to object to the school making her share a dorm with a male pupil. Thankfully, that's not normally necessary though, because she has people in her life who have parental responsibility for her. But the school violated her rights and the right of her parents to exercise their responsibility to champion her interests and uphold her rights.
I'd be absolutely livid and would not hesitate to file an official complaint with the school, asking for their risk assessment for the trip, for their relevant policies, and an official explanation for denying your child her legal rights to privacy, dignity and safety.