It's awful that teachers are being placed in these situations and it's no good for the pupils
Totally agree with this. My Dsis teaches in an FE college and was asked to teach a non specialist subject, she read up on it, spent so much time researching it and tried her best for the students. She even got her DD in as a guest speaker, as she did the subject at A level.
Then she was asked to do another subject in a very different area and she refused. She said it was short changing the students and embarrassing to be asked questions she didn't know the answer to.
She said she wasn't going to look like a fool and if they wanted to take any action against her for refusing, they could do so. From the other subject, she felt that on a few occasions the clever students (one imparticular) seemed to ask more questions and she felt it was just to show her up and then appear to snigger about it.
They let it drop and asked another lecturer to do it and she later heard some students in her tutor group complaining about how useless he was.
She and I had discussed this after she was asked to do the first extra subject and because they also come down on you, if your group fails to do well, I advised her what to say should it happen again.
From a HR perspective, I can't see what disciplinary action they could have taken against her, with the valid response she provided.
It's a sad state of affairs. In order to attract people into the profession, they need to make it more financially rewarding. Especially in the London area with the property prices whether you rent or buy.