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"I however do not think you can just decide, yep i'm female now, go into other female toilets come out an decide, nope male now then go into other male toilets. Something needs to be certain."
It's going to be enshrined into law that you can though.
I suspect it will be a protected characteristic under the Equality Act. People will be able to decide on a day-to-day, hour-to-hour basis what gender they are identifying with, therefore the hypothetical hairy-arsed builder could start the day as Brian and after work enter the female swimming bath changing rooms as Brenda.
If it hadn't been posted so many times before I'd be tempted to post The Life of Brian clip - talk about prophetic.