@letsallhaveanap
some people on this thread are like the 'Four Yorkshiremen'......'Well when I was three weeks overdue I ran the London Marathon twice, why would anyone want to sit down?!'
'Well when I was half way through giving birth I went round and did some DIY for my manager and then I cooked my family a Sunday roast. Then nipped back into the hospital to push the rest of the baby out. Childbirth isnt an illness!'
PMSL at this post ^ I agree with you. Some people on this thread are being ridiculous. Well I did this and I did that,so you should be able to do it to la la la!!!' So smug and condescending.
I also agree with @littlelionmansmummy
I should think that, like many pregnant women, she won't want to give her employer any reason to think she's not coping with her pregnancy. Lots of women struggle through because they're fearful of repercussions. Whatever the legalities of it, some employers will surmise that women who admit they are struggling with pregnancy may well struggle with meeting the demands of a job as a mother (however wrong that is). Women can feel very vulnerable in their jobs while pregnant and on mat leave.
Some employers are looking for ANY excuse to get rid of women who are pregnant/have kids. Not all obviously, but SOME ... Also, you get a hard time from some child-free women at work who love nothing more than a good bitchfest about the special favours working mommies get - you know like going home early if her child is ill. 
Honestly, as a mother of young kids you cannot win! You go back to work, and you're bashed and berated (by your fellow employees,) and you have every obstacle placed in your way to make life more difficult for you with 'well you CHOSE to come back to work' barked at you by people.., And if you become a stay-at-home-mom, you are labelled a lazy freeloader who expects her husband to fund her 'chosen lifestyle.' 
Going back to the original issue; only the most callous, thoughtless and selfish individual, would have a problem with a pregnant woman needing a seat.
@Mindthegap6 Complain to Virgin. And do it via twitter. Companies HATE that because so many people see it.
@Fevs
No pregnancy isn’t an ‘illness’ however because you are growing a tiny baby inside of your body and with that comes a lot of discomfort and sometimes pain and illness I think a little kindness and helpfulness is nice. Especially from huge multi million pound corporations!
Agree with this. And it's such a horrible, callous, mean thing to say 'you're only pregnant, not ill.' It really fucks me off! 
I agree with some posters on here that some remarks on here are very spiteful, uncaring, and bitchy.