The Workplace Regs is subject to an Approved Code of Practice, which is quite clear about separate washing and changing facilities for men and women unless they are fully enclosed cubicles, lockable from the inside and intended to be used by one person at a time.
ACOPs give practical advice on how to comply with the law, so following the advice is enough to comply with the law; you can still use alternative methods to those set out in the Code in order to comply with legal duties.
However, ACOPs have special legal status, so if you are prosecuted for breach of health and safety law, and it is proved that you did not follow the relevant provisions of the Code, you will need to show that you have complied with the law in some other way or a Court will find you at fault.
I’m not altogether sure how GLP could show that the words ‘man’ and ‘woman’ in the Workplace Regs mean something completely different to the words ‘man’ and ‘woman’ in the Equality Act.