They have a chain of command, and at each level decide where to focus their efforts (guided by existing laws, to be sure). Patrol routes are chosen, dispatchers must (effectively) triage calls, and officers make decisions like whether to assess a warning or a citation.
It sounds to me that addressing that problem at the enforcement level, as opposed to the legislative level, is the wrong place to address it.
So there is definitely at least some sense in which the police have discretion. The question is when and how much.
[1] I don't actually know if that's illegal in the UK; in the US it's a federal offense.
I think it's a little silly people pointing to the extremely limited "discretion" police have and saying they should have used it to ignore the law. That's not how police discretion works and in modern policing it hardly exists alongside it's counterpoint which is : neglect of duty.
Policing is not a job I'd want to do. Damned if you do, damned if you don't; and everyone knows how to do it better than the people actually doing it. But those people given the chance wouldn't do any better.