The Stinking Hellebore (Helleborus foetidus) can be considered both a native wildflower and a garden escape.

 For example I saw it recently around the margins of Hill End Chalk Pit in Hertfordshire. This corresponds to its natural habitat: scrubby, unimproved chalk grassland. The area is far enough away from human habitation that it's reasonable to suppose this might be the survival of a wild population.

 Then again it's one of those species that's considered "garden worthy" and has long been propagated for that purpose. It self-seeds readily where it finds the chalky conditions it favours. Gardens are more than likely the source of H. foetidus in the countryside near conurbations. 

 I often spot the bright green flowers alongside railway lines. No doubt the seeds have been carried in the wake of trains and found a suitable habitat in the limestone clinker. This is a wild species that was domesticated then "re-wilded" itself. So the distinction between 'native' and 'naturalised' is moot.

 The one seen above is in the grounds of the music school where Stinking Hellebores pop up all over the place. Probably planted as ornamentals at some point in the garden's history and multiplied of their own accord. Or perhaps they were here already on this chalk hillside when the house was built in 1859?