The Sea Spleenwort (Asplenium marinum) is a species of fern in the Aspleniaceae family.
This species has green, succulent leaves, and has lines of sori on the underside. As it's name suggests, this species is found on the coastlines, and is found around most of Britain, apart from the south east and east.
Sea Spleenwort - http://www.flickr.com/photos/qwertyqwertyqwerty/