Here's a good early find, a wild crocus.
Interestingly these grow around an ancient Neolithic Longbarrow not far from where i live. Normally recognised because its stamen are harvested from another similar flower, the saffron crocus. This one pictured is a real witchy version though, and much more deadly.
If the bulb, leaves or flowers are ingested it can cause respiratory paralysis, amongst other nasty symptoms. The toxin colchicine continues to increase as the seeds mature and is highest in the seeds themselves. Just a few grams can kill an adult human. I'd say this is in the top five of deadly plants in the UK.
As far as i know it's not a native but just out there now.