I eat yew berries, and have done for years. It is easy to spit out the seeds, so minimal risk of poisoning. Because of the sickly sweet taste of the berries you'd never want more than a handful anyway.
As for avoiding poisonous stuff, that's sensible, but the reason why the British eat so few natural things such as wild mushrooms is because of an uneducated fear of the unknown. True, some of them can kill you, and it's not a pleasant way to die, but many of them are easy to identify and are quite tasty, so why miss out? A little bit of care and knowledge is required, but armed with them it's quite safe.
As for blaming the yew tree for people being poisoned by eating a fungus growing near/on the tree, is there any more information on this? Was it a normally edible species that was eaten? Or did they not know what they were doing? Seems a bit doubtful to me, especially as I don't recall ever seeing a fungus on a yew tree!!