[wolverine876]

Why not introduce furry mammals that don't need a large expanse of nature, but can adapt to and live directly in the urban environment. Something that could live in all sorts of corners of the city, even underground, and consume unwanted food from humans ...

I'm joking a bit, but I'm serious. Why are beavers better than rats?

[stubish]

They are larger and build damns and many other differences, so they fill different niches. The idea is to restore the pre-industrial ecosystem as best we can. Maybe their natural land management will improve the quality of the wetlands, or maybe it will cause flooding of nearby human occupied land and cause them to be quietly removed. Maybe they will become prey to wolves and lynx, or maybe they will need to be culled because those predators probably won't be released in London.