BBC article seems quite straightforward on this one.
They went to some lengths to look for a transwoman actress for the role but couldn't find one good enough. So they cast a man. What else were they to do? By the sounds of it the character also appears pre transition, and is actually male, so it would be nonsense to cast a woman.
I did initially have some sympathy, because trans actors may find it difficult to get traditional male/female leading roles due to not passing well, so some evening up seems fair enough. But going by the BBC article it sounds like significant efforts were made to find a trans actor and noone good enough appeared. That is entirely fair I think - if you were casting a disabled part and no good enough disabled actors applied, you'd be reasonable to cast an able bodied actor.
And TG campaigners do indeed object because casting a male as a male character is too realistic. Couldn't make it up. Are they trying to say that transwomen who don't take hormones/have surgery are not trans enough to appear on stage even in the role of ... a transwoman who doesn't take hormones or have surgery? What else do they think the difference between a transwoman and a man in a dress is? I thought it was supposed to be purely in your head, in which case who cares, nobody expects an actor to think like the character, they just have to act like them.