You sent me scurrying me off to a textbook I used when I first entered university to answer this question.
For many years the standard textbook used by botanists to identify the British Flora was a work called "The Flora of the British Isles" by Clapham, Tutin and Warburg. This book ran into many editions and was eventually succeeded by another bigger textbook called "The Flora Europea".
According to Clapham, Tutin and Warburg (CTW), Urtica dioica has always been a native species in the UK. It is known to have existed since prehistoric times in the UK.
A more recent book called �Flora Britannica� which has much background information and folkelore on flowering plants supports CTW. This book mentions areas around Salisbury for example, which are dense with nettles since the remains of an occupation that ended 1600 years ago - the nettles are still thriving on phosphates and other minerals in the wake of human and agricultural colonisation.
There are also various villages in England such as Nettlebed, Nettleham and Nettleton, some of which show evidence of having existed prior to the roman occupation.
So yes, Urtica dioica is a native British species. However, that's not an end to the story.
(Cont)