I don't think this can be anything other than Cancer pagurus, the Edible crab, lower element of the chela (pincer).
There are a few smaller spp. with dark tips to the chela, but the size (judging from the gravel - measured size of specimens always helps id) rules them out. Also the outline of where the black joins the rest of the shell is right for C. pagurus.
If you collected the fragment, take it to a fishmonger's shop and compare it with the crabs on his slab.