short stories - 1970's- 1980's children's SF anthology (British?) identification

I've been trying for years to find a children's/young adult science fiction collection that I read growing up in the 1980's in the UK. It wrecks my head whenever I try and look it up. There were a few stories in it, three of which I can (vaguely) remember:

First (?) story: aliens destroy the world by giving a disaffected young outcast the powers to do so.

Another story: This guy is tending, like, alien sheep-like creatures that secrete some valuable nutrient goo, but he gets bored and starts beating them or killing them, and then they gang up on him and kill him by forcing this nutrient goo down his throat.

Another (final?) story: young boy start slipping in and out of a coma, when he's 'under' he finds that the regular world is an illusion, and is in fact a nuclear wasteland that was destroyed by an alien race in a war thousands of years ago. Out of guilt, the aliens have stuck around since then, maintaining a kind of psychic projection for the benefit of the poor sickly human race. The boy, and this old geezer in the same hospital are members of this alien race and the boy has to decide whether to pick up the baton, and devote his life to the illusion. The story ends with a hospital nurse telling his parents he's gone into a coma from which he'll never wake.

So there you go. A fair amount of information but virtually ungoogleable (and you get some disturbing results when you do google it). I'd love to know what the anthology was called so I could get a look at it. Anyone know? Or know another site that might be able to answer it?