I wouldn't have thought of a polytunnel for giant thistles
I did grow artichoke plants the first year but soemthing kept eating the leaves even when they were a foot high. I found that annoying considering I've got one slope where wild thistles grow superbly with no sign of munching....
You'll find me promoting the following at every opportunity: there are cold hardy variants of pecans, hickory nuts and lots of other nut trees one cn grow..if you have the patience to plant stuff for future generations. Monkey puzzle nuts are highly nutritious too. The website name is on my other PC, though.
Also look at honeyberries for novelty and tolerant to -40C
Also consider some citrus - just need to keep it frost free over winter as opposed to tropical. I'm virtually self sufficient in lemons and limes but I think my grapefruit is on the wrong rootstock and I did lose my only orange some years ago.
There's a plant place on anglesey that claims to grow coffee and tea bushes in unheated polytunnels among other stuff and tea is grown in devon. A dessert grape will always go better in a polytunnel and with careful pruning can act as some summer shade