As far as toxic to touch I don’t think I am aware of anything in the UK you are realistically going to come into contact with.
Giant hogweed (related to hemlock, looks a bit like parsley). Looks like normal hogweed, but far more toxic. Won't kill you from touch alone, but not a great idea to handle it without gloves. Found all over the UK.
Variants of hemlock are also abundant in parts of the UK. Quick google turned up a story about it growing near a school. Obviously parents panicked, which is silly because it grows all over the place. Hemlock water dropwort is the super poisonous one, found all over the place but I doubt most people'd realise. Left is hemlock and right is cow parsley:
Obviously the person holding them is touching the hemlock with bare hands, which is a bad idea.