This is the first time I've seen a company differentiate between sex and gender. Their statement from the article:
"Having single sex changing facilities is in accordance with the Equality Act 2010 and we ask that guests use the changing facilities appropriate to their legally defined sex.
"If guests wish to use changing facilities appropriate to their gender, we will always speak to them to see what options are available to them, while taking into account and balancing the interests of other guests. We do offer all of our guests the option to use a private changing facility to ensure that all guests and visitors can use the spa."