I saw this rock on the beach in Hunstanton, North Norfolk and tried to find out, via the www, how it came to be.
The most likely explanation is the holes in the stone are caused by burrowing marine animals: the large holes are possibly made by a bi-valved mollusc called the Flask Shell, Gastrochaena dubia Pennant. or maybe a Piddock. The Piddock’s shell has a set of ridges or “teeth”, which it uses to grind away at clay or soft rock. The shape of the hole made is due to the rotating motion of its shell as it grinds the rock to form its burrow.The smaller borings are most likely the result of attached mud-tube-living marine worms like Polydora ciliata in which the acidic waste products dissolve the stone.