I have to take public transport in rush hour tomorrow and am not looking forward to it! I don't live in London so we don't have baby on board badges. People may wonder if I'm pregnant in my huge maternity coat (I'm 30/40) but I could easily just be on the round side so most people wouldn't dare ask. Most people who are lucky enough to get a seat tend to avoid any eye contact with the standers so that they don't notice if someone needs the seat more than them. Also, a lot of people sleep, read or listen to music so it can be hard to communicate.
Would you dare to ask someone/ the bus at large or stand for 2 20 min bus rides, facing the risk of squashing your bump as people get on and off?