The priority seats generally have little labels saying "please give this seat up to someone with limited mobility/ less ability to stand" or similar. So anyone can sit in a priority seat, but they should vacate if the seat is needed by someone. You don't leave them empty in case somebody needs them!
So yes when I was pregnant I would ask the people in priority seats if they would let me sit down. If they said "no sorry I need this seat" then I'd ask someone else/ hopefully somebody else would stand up, or I'd get off.
In two pregnancies commuting in London only once was I offered a seat without asking! Every other time I politely asked and somebody stood up. People generally travel in their own little bubble and don't look at other travellers to see if they have a bump/ disability. I would ask the people in the priority seats if they didn't have an obvious disability/ elderly, if I thought they wouldn't be able to stand up I'd politely ask somebody else. You're only asking, they can say no.
Obviously they might have a hidden disability, but if you don't ask you'd never get a seat, and more likely they were just in their own world.
Personally I would much rather someone just asked for a seat if they needed it. I hate having to work out how old is old enough to offer a seat, whether that is a pregnancy or just a big tummy, whether that person looks unwell or not. It can offend people if they think you are incorrectly saying they are weak/ old/ pregnant....
Didn't a comedian have a joke "I'd rather see a pregnant Lady stand up than a fat lady sit down crying?"
So the pregnant lady was perfectly reasonable to ask for a seat. And you would have been perfectly reasonable to say "I'm afraid I need this seat ( could add due to an injury/ illness/ disability if you wanted)". She can ask somebody else, or get off the train.
Having got up, it was unreasonable of you to criticise her. Either say no, or get up. Don't get up reluctantly then criticise her. She just asked you, she didn't demand. you could have said no! I would have felt irritated in her shoes, it's like you are saying she was too forceful and you were unable to say no