Ticket offices closed four years ago.
Station staff kept a supply in the station office but Customer Services stopped taking orders for replenishment. You now have to order one on the phone or online.
I think this partly has to do with data gathering. We also have a "give up your seat" badge for people who have hidden disabilities. If you have to give personal details to get either badge TfL can then send bulletins about improved access facilities etc. and roughly calculate how many people with extra needs use the badges.
Also, cost. Sometimes people would ask for two (we have 2+ coats, don't we?) so if you order from CSC they will send one. Though postal charges are involved so I don't know really how much is saved by posting the badges.
London is a rude SOB so my advice is to simply say, "No, I'm pregnant". The Underground is bringing in colour coded seats with pictures of pg females, disabled and senior logos on them so be sure to try and find one. Be assertive enough to turf people out who are not entitled.
I abuse my authority sometimes and ask people to give up their seat. Got into a few scuffles but my dad has a hidden disability so I know what it's like to be ignored on his behalf. I have travelled with him in my regular clothes, and he's too shy to ask, so I say, my dad can't stand for long, could you give up your seat? Usually it works.
(I'm a tube station supervisor)