Here is my infallible way of getting a seat on a crowded tube train if you are unable to stand for any reason be it pregnancy, sprained ankle or, like me, a propensity to faint in crowds.
Stand in front of a priority seat or, a seemingly fit person.
Smile.
Ask in loud but firm yet pleasant voice.
“Excuse me, but would you be so kind as to let me sit down?”
This method has never failed.
Occasionally I might be ignored by the subject in which case a one of the human beings sitting either side will always provide.
Saying, “Oy, this is a priority seat, why are sitting there? Gisit to me!” is unnecessary.
Glaring and looking pathetic rarely works.
Being a drama queen (or king) gives you no brownie points.
If, for any reason, you want to sit down a polite request is all that is needed.
Just saying.