Common Moorhen, Gallinula chloropus
When I photographed the birds shown above and below on a Christmas visit to London in 2009, they were considered to belong to a world-wide species, Gallinula chloropus, with the American colloquial name of Common Moorhen (simply "Moorhen" in the UK.) In 2011, the American Ornithological Union split the world-wide species in two, with the Old-World birds retaining their original colloquial and scientific names, while the New-World birds were given the scientific name Gallinula galeata and the colloquial name Common Gallinule, adopted from an earlier American name for the former world-wide species.
It was a cold Christmas season in London in 2009, and the pond at Clissold Park in Hackney, where I photographed the birds shown above and below, was frozen over for most of the time I was there--hence these pictures of Common Moorhens on ice.
Here's a London Common Moorhen swimming in summer, July 2013, at the gardens of Kenwood House on Hampstead Heath.