An uncharacteristically large mushroom growing on the grounds at Roehampton.
As pointed out by the first comment, this is the Common Inkcap (Coprinopsis Atramentaria) mushroom. It is secreting a liquid that was once used as ink, and while edible, becomes very toxic when combined with alcohol (even if only a small amount of alcohol is consumed three days later). This, however, has not been recorded as fatal.
I love Wikipedia.