Walking around London yesterday morning for an hour (with a step ladder!) we managed two carrier bags of tasty ripe plums. I also ate some orange things that I have no idea what they were but they tasted fantastic. There's raspberries, blackcurrents, blackberries and even (in an abandoned lot) tomatoes which my friend ate. I missed the mulberries this year but in a couple of weeks I'll also have some apples and avocados to add to that and maybe some persimmon. And this is all stuff that'd be on the floor and/or rotting later without people blinking an eye. So many fruit trees in London! And that's not even mentioning the mint, huge basil tree-bushes, rosemary, lime trees and countless other herb lying in clear sight.
There are plenty of poisonous plants growing all over the world. Not eating them is personal responsibility.
Several of the ones you've possibly heard of from Harry Potter or general folklore (monkshood, wolfsbane, deadly nightshade, mandrake) are poisonous and wild (sometimes common) in Britain.
I've known daffodils, foxgloves, rhubarb leaves, holly berries and others were toxic since early childhood.
