Darren Obbard points out that this is an unusual plant for the UK:
"Bizarrely, this individual is monoecious (hermaphrodite) suggesting it is from the polyploid lineage (Mercurialis ambigua L.fils. -- or sub.sp. ambigua, depending on classification) - This most definitely should not be growing in the UK as it is native to Iberia/Morocco!"
(British plants would normally be either male or female individually.)