If you would like to eat outside in London, you have a varied choice. From a summer cream tea in Hyde Park to a hot-dog on a frosty night after the theatre, London can provide lots of choices for easy outside eating.
In the evenings when you are wandering around London, especially in the winter, you will see street vendors selling hot cooked sausages, served with onions and tomato ketchup in a bread bun. Whilst these can look and smell very, very tasty.... you should know that these street sellers are invariably un-licensed, have no chiller facilities for storage of meat and the street hot-dog sellers are likely to be arrested for street trading without a license. Armed with that knowledge - do you still want one of those great smelling hot-dogs? :-)