you'd have to hope that a court would side with Thea. But if there was a time, for eg, when she went in, the trans people kicked off and said it was because she was there? I'd wager he would be able to do that. Unless Thea could show that she and her fellow GC friends had been going there at the same time as the trans patrons for years and years with no problems.
I would guess the LL would get away with that, unfortunately.
ETA. It is a LONG time since i worked in a pub, but we had 2 people try to get us to rescind a ban by getting lawyers involved. We were very clearly, say, a City pub with banners inside, scarves, photos of players etc etc. Fairly close to the ground On derby day a, say, United (to be clear: it was not in Manchester but a city with 2 teams) fan came in singing their terrace songs, wearing scarf, hat and shirt etc and generally going up to locals and getting in their faces. And one of the city fans finally threw a punch.
So the city and the united fan were kicked out - the city fan could come back after that day, but the united fan was banned. He found that unfair, but he provoked and provoked. The ban stood because the LL couldn't guarantee his presence wouldn't lead to a fight (in a well established City pub)
The second was a chap who was deffo a few french fries short of a happy meal, who used to hassle our weekly blue-grass band - who all worked for a (then) well-known eco charity. It was clear, they had 2 banners and sang songs about saving the earth and how bad nuclear power is. And this guy used to just shout abuse at them and nuclear is great and all that. Banned. Lawyered up, and his own lawyer told him to knock it on the head.
I think a LL trying to "keep the peace" - no matter how spurious the grounds - may win if there is evidence of previous trouble or definite threats? IANAL