This is one of the spider crabs, characterised by long legs and pear shaped carapace. There are many different species of small spider crabs, and often the various seaweed and other growths on them, disguising them, makes species identification difficult.
Having taken your photo and closely "mapped out" the edges of the carapace, the outline eliminates some species and makes possible others.
The eyes are prominent, with only a hint of the front nose points, the rostrum, which in this case is very short. The carapace appears heavier than some species, and is rounded towards the rear, whereas some species are quite square in shape. There appears to be what could be quite a prominent spike, or sharp corner, seen below the right eye, which is obscured on the left by weed. The legs are not thin and spindly as in some species, but seem quite robust. Unfortunately the claws are being held underneath the crab, out of sight, which would have been good to see.
Given these characteristics, and the location you found the crab - on the shore on a Norfolk beach - again some species could be eliminated. Accepting it is sometimes difficult to identify these small spider crabs without pulling weed etc off them, there one species which might fit with what we've got - Hyas coarctatus.
Mind you, I could be totally wrong and it turn out to be Inachus dorsettensis. Here lays the difficulty in identifying a disguised crab!