If they're sunbathing in March on Shepherd's Bush Green, reason is a questionable faculty to appeal to. But I do like seeing cops ask nicely, as opposed to "you have 20 seconds to comply" or whatever they're going to end up saying in Alabama as an immediate prelude to watercannonading the locals.
Perhaps tutting played loudly over the public warning systems would be more effective? Just someone's mum tutting and occasionally sighing. This could be an incredibly effective wide-area suppression system in the UK.