It's one of the spider crabs. Going by size, shape and general appearance this is Hyas araneus, also known as the Great Spider Crab. It is found on all British coasts but more so around your way than down here in the southwest, which suggests a preference for the cooler waters further north.
These crabs often have growths of seaweed and sponges making them difficult to see at times when alive on the seabed. This particular specimen must have been at this size for some time, judging by the growth of barnacles, tube worms and little saddle "oysters" on its surface.
Similar species are Hyas coarctatus, (broader than araneus, and with more pronounced corners behind the eyes), and Maja squinado, (much larger, with the carapace up to 20cm long, which is very spiny - hence its common name, Spiny Spider crab).