Violet webcap (Cortinarinus violaceus)
Webcaps are a family of fungi that contain some pretty poisonous species and, whilst they aren't all poisonous, foragers avoid eating them as a whole to avoid any accidental problems. Violet webcaps, like all the Cortinarinus family, have rusty, red brown spores so always check for these. They also lack the lovely flowery/fruity smell of Wood blewits and are a deep violet colour in cap and flesh.
Also watch out for the Goatscheese webcap (Cortinarius camphoratus) and the Gassy webcap (C. traganus) both of which smell horrible.