Can't help with locations in Cumbria but I generally forage on the hoof, so to speak. As in, if I spot brambles, wild strawberries, plums, apples, pears and such like, I'll have a little nibble of them as I pass. I guess that's one advantage to low level walking (doubt you'd find much to forage up a mountain).
I wouldn't dare touch mushrooms though - too many deadly ones that look similar to perfectly safe ones and I don't have the knowledge or skill to tell the difference.
I once happened upon a sprout field near where I live.... that was my evening meal's veg sorted out then
Edited to correct my wrong spelling of pears (oh the shame
)