It's important to be polite to the police officers if they want to have a word with you.
Some peoples' first reaction is to be violent or rude if questioned by an officer (I am told that Hampshire Police are extra-annoying and will question anybody

). If you just say, "I'm a railway enthusiast just going about my own business" they can't argue with that, although when on stations (especially in busy stations) it's best to ask permission.
At busy times, it's also best just to avoid big stations as there will most probably be officers on duty who will get suspicious of someone taking pictures. I personally tend to avoid big stations anyway, unless I so happen to be needing to travel through them on other business. Even then I only may grab a couple of pictures with my camera if I have enough time in between connections.
I know that not going out and staying locked up in our homes and spying on everyone else is letting the terrorists win, but as long as you just don't look suspicious and are careful then it'll be ok to go out and enjoy the hobby. The best way to get over the questioning and suspicions is just to go out there and be honest and polite to anyone who questions you.
But maybe until things have calmed down a bit it would be an idea to leave visiting London (and other major city) stations for a little while, so as not to bother the officers on patrol and be embarrassed with a search.