I think they can probably defend their changing rooms policy up to 16 April (SC), as guidance fom the EHRC used to recommend that approach.
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Would they though?
The Health and Safety legislation from 1992 mandates single sex changing rooms in this case. TransActivism has only become a thing in the last 20 years, so there would have been a period of established single sex usage.
They might have an argument that a man with a GRC qualifies, but there is enough evidence to show that Upton could not have had a GRC at the time, so they would know he didn't qualify. But IANAL.
It's not entirely relevant but when a relative qualified as a Dr about 15 years ago, I am sure that the records said Man or Woman, and not Male or Female.