Well the guidance is going to say the same thing as the interim guidance which is extremely clear about schools and says this:
Schools in England and Wales must provide separate single-sex toilets for boys and girls over the age of 8. It is also compulsory for them to provide single-sex changing facilities for boys and girls over the age of 11. The law in Scotland requires schools, irrespective of pupils' age, to provide separate toilet facilities for boys and for girls. Toilet cubicles are required to be partitioned and have lockable doors. Pupils who identify as trans girls (biological boys) should not be permitted to use the girls’ toilet or changing facilities, and pupils who identify as trans boys (biological girls) should not be permitted to use the boys’ toilet or changing facilities. Suitable alternative provisions may be required.
So they're breaking the law if they don't do this immediately. There is no justification for not following existing law and EHRC interrim guidance.
An interim update on the practical implications of the UK Supreme Court judgment | EHRC
Personally I would copy this to them with the link and ask them to confirm that they intend to continue to break the law and refuse to act according to EHRC interrim guidance by allowing male bodied children into spaces labelled as single sex for two months until the new guidance comes out.
I would ask for information about their insurer (so you can write to them with a copy of this response), and ask if they have informed their insurer of this intent. Note that if any girl is harmed as a result of their policies, given they are breaking the law it is your understanding their insurance company is unlikely to pay out however as you are not a lawyer you advise them to ask their insurer this question directly.
Then I would ask Sex Matters if they can help with free legal advice and a lawyer's letter. This is a completely unacceptable delay in complying with the law (and protecting girls).
This is putting activism above child welfare. They can be TRA activists in their own time, it's breaching their safeguarding responsibilities to ALL children do it in school. Poor kids in that school. I wonder what the homeschooling / attendance rates are.