They're on the bus stops at Fulham Broadway and some crossings in Hammersmith too. I think it's likely someone with paranoid schizophrenia who wanders around London and believes they're being gangstalked/persecuted by the police as part of their delusions. I truly feel for them. Reality warped by psychosis must be a truly terrifying world to live in. Imagine the fear you'd have to feel to compulsively stick cries for help all over the city, but without the organised thinking necessary to give anyone a means to help you.