I heard this discussed on the Sex Matters podcast.
Their guidance basically says that the trans person should be allowed to use whichever facilities they want, otherwise it could amount to harassment. And as was pointed out on the podcast, they say the guidance has been developed with help from legal and discrimination experts (including Robin Moira White 🙄) so it sounds authoritative, especially to orgs who don't have knowledge of what the law actually says in this area.
Sex Matters have reported them to the EHRC. It's pretty outrageous that trans lobby groups are continuing to misrepresent the law in this way.
www.transinthecity.co.uk/ehrc-guidance