"The City of London Corporation is responsible for ensuring legislative compliance for imported animals into the whole of Greater London."

From https://www.cityoflondon.gov.uk/serv...s/default.aspx

Further reading:

"Since the 1850s, the City of London Corporation has played an important
role in the development of animal health and welfare. Its responsibilities for
imported animals and the prevention of disease extend beyond the City to
encompass all Greater London, some 610 square miles."

From https://www.cityoflondon.gov.uk/serv...e-Brochure.pdf

Probably comes from historical port authority remits?

As for being out of their patch, the City of London have stuff all over the place, for example Hampstead Heath, Epping Forest and Burham Beeches are all City of London.