Very common all round Britain on coastal rocks, frequently growing over the black crusts of Hydropunctaria maura. Typically placoidioid, but the lobes are often discontinuous on rough surfaces

References: 
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Irelandd
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species