Another feral species doing well in the UK is the Mandarin Duck (Aix galericulata) . Originally from Asia, they were kept as ornamental pets (for obvious reasons) but many hopped the fence to set up a wild population in the UK.
They have been successful and as tree-nesting ducks occupy a niche that seems to have been unexploited in the UK. In fact deforestation and other threats have damaged the Asian population to the extent that it is sometimes said there are more mandarin on the Thames than in China. This handsome fellow is the drake but the female whilst having subtler colours is just as beautiful in my opinion.