Red-eared Terrapin (Trechemys scripta elegans)
Neither having owned any aquatic pets, nor knowing anyone who had done so, I was quite unaware about the presence of Terrapins in some London ponds and shallow lakes, so I was quite surprised to stumble across these in a pond on Tooting Common, the result of over-ambitious pet owners finally realising that cute turtle-like creature crows into a sometimes bad-tempered adult needing considerable space and care.
More on the Terrapins can be read and viewed here:

The red-eared slider (Trachemys scripta elegans), also known as the red-eared terrapin, red-eared slider turtle, red-eared turtle, slider turtle, and water slider turtle, is a semiaquatic turtle belonging to the family Emydidae. It is a subspecies of the pond slider. It is the most popular pet turtle in the United States and is also popular as a pet across the rest of the world, and is the most invasive turtle.