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My first thought was a Dierama species, but the flowers don't look the right shape. Some kind of Penstemon? I don't know of any growing in water, though.
Google finds just one UK garden centre selling it, but that company's photograph is a dead ringer for what I saw (unlike other photographs from Ameria).
PS: The Jepson Flora Project tells me that there are wetland Penstemons, but Google is reluctant to give me a species name.
Try this page from the United States Department of Agriculture, http://plants.usda.gov/wetland.html , 21 wetland Penstemons are listed.
Lat/Lng: 53.0166, -2.2188
OS grid ref: SJ854466
Growing in wildlife pond in municipal park.