Police spotting (junction watch)

They might be able to cross where they like but is there no offence for doing something stupid and causing an accident?

Like, if I ran in front of a bunch of cars all day playing chicken - am I really free to do that? Because it seems the peds in London are free to do that so maybe I should implement my own traffic calming measure - by sitting in the street.

If you were doing it persistently, all day long, you could probably get done for obstruction of a highway as you'd be easy to get catched. All the millions of peds in London doing the same thing aren't really doing it in such a way that makes them easy to get catched, though, and there's not really any way of proving each individual is doing it persistently, all day long.

But in normal traffic conditions, you should be prepared to slow down or stop at any time due to unexpected situations. In the highway code, innit.