Americans call it the Hoary Rosette lichen but the Brits don’t appear to have a common name for this pretty little lichen, Physcia aipolia. I found it flourishing on wooden fence railings alongside the local coastal path so it obviously thrives in an exposed and salt-windswept location. Officially, it is usually found on the well-lit (I presume that’s sunlit rather than under lamp-posts) nutrient-rich wood of all manner of trees, their twigs and bark.
Its thallus (a plant body that doesn’t have stems, leaves, roots or veins) is foliose (has a lobed, leaf-like shape) and its apothecia (the cup-shaped fruiting bodies) have white rims, with dark brown or black centres. Physcia aipolia is widespread and common in Britain, though it does seem to prefer the slightly warmer and perhaps wetter climes of the south and west.