Mallards are very common ducks, and I feel sorry for them. They often get overlooked, despite their gorgeous plumage (the male's emerald green head is spectacular in the right light). I was unsure if the male here was a hybrid (or sometimes called a "manky mallard") as the bill appears to be blue instead of the normal bright yellow. I did some research and it turns out occasionally, males have this colour bill (as he appears to otherwise look like a pure mallard).
Widespread, but needs some kind of body of water or wetland nearby.
Found this pair on the University of Essex campus grounds.