Steve, I haven't taken time yet (my kid is saying "come on, Mom, let's watch something!") to explore how reliable this article may be, but I'm gonna post it anyway.   Don't know about in the UK, but feral domestic cat populations are significant throughout the USA, and that is in addition to the number of cats intentionally fed, if not turned into pets, by households in both rural and urban settings.

Cats and wildlife: A conservation dilemma