Still bothered by the sex discrimination angle. IANAL, but as I understand it, you compare the complainant to a broadly similar employee of the opposite sex to assess whether there has been unlawful sex discrimination. But this situation wouldn't have arisen if Grant had been a woman so it's hard to compare. It seems to me he was sacked not because he was a man but because of the public reaction to him being a man in what was perceived to be a woman's post. That may be a distinction without a difference, but I think that if a woman had been appointed to a role which her sex made her obviously unsuited for, and there was an outcry, you could argue that she would also have been sacked. If no-one had mocked the appointment, Jason Grant would still be in post.
In an imaginary scenario, an employer appoints a male toilet attendant for the women's single sex toilets. The users complain about the attendant being a man, the attendant is sacked and the role axed. In that scenario, the employee has been sacked because of his sex, but the same would have/has happened to the woman attendant in the men's single sex loos. They've both been sacked for their sex, but their sex is the reason why they are unsuitable. You'd have to argue that it was unreasonable to think of this as a role only for a female or a male (depending on which toilets) - extrapolating from the reasoning behind the provision in the EA2010 for lawful discrimination on grounds of sex when recruiting. So the employer has messed them around by employing them in that role in the first place and should cough up, but I can't see how it's sex discrimination.
I'm probably wrong, but I would love a professional to explain to me how it works, if any employment lawyers are reading...