I greatly appreciate the women who wear these badges: https://tfl.gov.uk/forms/23479.aspx because it makes it unambiguous. It can honestly be hard on a packed London underground train to tell the difference between a pregnant woman and someone who is simply overweight.
If they're actually just fat, or if they don't consider themselves old, or if they don't speak English, or they're upset at my presumption they're weaker than me, or they're mentally ill or whatever, no problem.
Of course, if there are people between them and your seat, there's a chance someone different will take your seat. So it's not an infallible solution.
I'd only been in London about 2 days, I jumped up like a good boy scout, conscious of the fact I was in the "priority seat" (for those outside of London our tubes have the nearest seats to the door reserved for those less able or up the duff).
This lady burst into tears. I tried to apologise, then just got off and walked.
Since then I've firmly put the impetus on those in need to ask.
Also, pregnant women tend to hold their bellies. Overweight people don't. And if you accidentally offer your seat to someone who's chubby and has indigestion, well just tell them they looked like they had indigestion.