Confidence: It's likely to be this, but I can't be certain.
Notes: Although this starfish is large and conspicuously spiny I don't think the form and arrangement of the spines is right for Marthasterias glacialis. No other species of British starfish seems to fit so that leaves common starfish. There were large numbers of large individuals like this one in shallow water at this location. These must be near the upper size limit for common starfish?