A tricky one... The answer is that Rosebay Willowherb Chamerion angustifolium appears to be both native and introduced.
It was a rare upland species in the British Isles until the 19th century, when it dramatically changed its behaviour. Perhaps with the introduction of genes from elsewhere in its range (N. America or Scandinavia, perhaps), it became a vigorous pioneer of disturbed habitats.
It has now expanded its range to most of mainland