I never ever take a seat on the tube unless there are several free, and I am always happy to stand up if there aren't many free usually.
My epilepsy has been super bad recently - as in two or three seizures overnight - I'm fucked basically (there is no way to say this nicely; I'm a mess). I've had to take the tube a couple of times and on these occasions I have sat down; twice I used the "priority seat".
The second time, I had a pregnant woman (with the badge, but not really a bump yet) ask me to move. I explained I wasn't well and it caused a bit of a shit storm... I was called a few choice words and not wanting to deal with it stood up and gave her the seat, and someone else gave me a normal seat.
However, now wondering whether I should have moved - is there any "rules" on who gets priority of the priority seats on public transport?