Letting dogs run free just anywhere in the city is against a London bylaw. There are off-leash parks for that purpose. Still, one often sees dogs running after balls in city parks. My granddaughter laughs with loud delight whenever she gets a chance to watch one of these dogs in action.
One time the pooch in question, hearing Fiona, brought the ball right to the little toddler, dropping it at her feet. The dog brought more than a ball to Fiona, the dog brought her "joy".