@Melian I haven't got my fit to fly yet, but I'm definitely not expecting it to say I need to be in a wheelchair - that sounds very excessive unless there's some other health condition that might affect you. But you could just say no to a wheelchair if offered one.
Popped into my GP practice earlier and asked about fit to fly letters, they have said it will cost £20-25 and might take up to two weeks, which isn't too bad! I'm not even sure I'll need it as I don't think I look as far along as I actually am, but would hate to have an issue at the airport for my journey home when I'm right on the edge of when the airline allows travel without a doctor's note.