This beauty is definitely a Mallow (Malva sp.) but it seems paler than the Common mallow (Malva sylvestris), whose flowers are usually a much deeper pinkish-lilac with even darker stripes.
I found it growing on Penarth’s rail trail, a railway line to Barry that fell foul of the Beeching cuts in the 1960s and has since been converted, in part, to a much-used walking and cycle path. The trail is edged on both sides by houses so this plant could very easily have flitted over a back fence or been dropped as seeds by birds. Whichever, its flowers are a very pretty addition to the foliage that lines the trail.