I don't think fly argic is illegal (I'm from the UK btw) and I don't think it's poisonous to the point where you eat a few and you die. People eat them and get nice trips off them so yeah they are legit.

As for the ammount needed, say for a proper trip you'd need to eat 2g dried, but the heat from smoking destroys most active chemicals so you'd need to smoke significantly more than 2g to get properly tripping.

For a nice relaxed giggly type feeling I recon a joints worth would work okay.