N. peltata looks like a fancy Water Lily but is actually a member of a different family: Menyanthaceae. Its closest relative in the British Isles is Menyanthes trifoliata (Bogbean). It 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 birds 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.