Hi Marcus,
A few thoughts occur on reading your post, oak trees are probably the least difficult tree for under planting, although water requirements will always be high.
Leafy veg tend to do well when shaded but will need more watering.
Most fruit plants are originally woodland edge plants so will get by at the drip line of the tree canopy. The crops will be smaller and later however.
Plum trees are greedy and i find suckers rising 25 ft from fan trained ones, but suckering is less likely on standard trees unless they are weakened.
Alpine strawberries love a shady spot and can self seed around if left to their owd devices.
I’ve had a good crop of leeks from a heavily shaded bed before, and good brassicas close on the south side of a huge conifer hedge, that did need some digging as i put a trench along the side of the bed to sever the feeder roots.
Hope that helps,
Tristan
