This plant is native in East Anglia and in fairly well populated areas such as those that border these meadows, the plant could well have been introduced. However it is a haven for bird which are famous for distributing our flora so it could have arrived naturally.

Although Stace reckons this is a rare plant, I think what he means is that it is only truly native in a few places but it is actually often recorded in the British Isles and is particularly widespread in the north west and south east of England. It is present in a few sites in Scotland, Wales and Ireland too so it isn't really rare in the accepted sense of the word.