Recently I got a chance to ask a park worker about the turtles seen in the nearby pool, and to my surprise she revealed they're actually treated not as a part of the wildlife to take care of but as pests.
[here's a better photo made in the same park by someone else]
The story behind is that when the Ninja Turtles series were ridiculously popular 20 years ago terrapins were introduced to many parks in London. Alas, that didn't go well as soon it turned out they bred too fast causing threat to other species. Since that moment, no new turtles were released into pools. Even more, they are still looked for and removed every time ponds are dried for regular maintenance procedures.
Despite these measures the capital turtles have been struggling for two decades already, so far coping with the hostile attitude as well as with the unusual environment. Obviously, you can't expect less from ninjas.