Error 2p ?

To be that much thinner, some metal must have been removed, so it's not a brockage or die cap of some kind. It is either a "split planchet", or a PMD lathe job. Some consider split planchets to be mint errors since the ultimate cause is a mint-made defect (a gas bubble deep inside the planchet creates a flaw the coin can split along) but since the vast majority of them only manifest themselves long after the coin leaves the mint, when the coin is dropped or struck at just the right angle, I do not consider them truly "mint errors". So either way, it is neither particularly rare, nor an error.