One of the inkcap fungi, possibly the Scaly Inkcap (Coprinopsis variegata) which grows on decomposing hardwood. It won’t kill you if you eat it, but the fruiting bodies of the bitter-tasting mushroom contain the Antabuse-like chemical coprine, which causes a pronounced emetic reaction when consumed with alcohol.
This group were growing on old, rotting wood forming part of a stumpery in the back of the garden. There were a good number more to be seen but these were the most photogenic.