This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
(The number of truly native plants is not as great as that of the aliens, casuals, colonists, denizens, etc that have found a place in the British Flora. Natives of other countries have become established, or have existed for a longer or shorter time in the British Isles, and in the same manner British plants have found their way into other countries where at one time they were unknown. The exact status of many plants is uncertain, and there are some species that may be truly native. There are nearly two thousand alien plants that have been recorded from the British Isles by Mr. G. C. Druce. Hardly a tithe of these can be enumerated and briefly described here, as the others are of very uncertain occurrence. By reference to Mr. Druce's valuable list, however, those who wish to know the extent of the alien flora can ascertain the species that have been found. Mr. Dunn's alien flora gives some particulars also as to a large number. A new alien flora by Mr. Druce will constitute a comprehensive guide to these plants.)