On land where your tenure is unsecure, you want food that requires a minimum of money/effort. So:
scatter a manky pot of raspberries or strawberries (you can make a herbal tea from the leaves of the former)
toss your applecores (some of the best apples I've tasted have been from such seeds, chucked by the wayside)
...and some hazelnuts
planted ornamental almonds can offer tasty nuts (if too almondy, then too much hydrocyanic acid) - but you may need adjustable spanners to crack them
learn to recognise Ground Elder and know it from poisonous relatives - it's great cooked as spinach, or young leaves in salad. Don't introduce it to your allotment though!
plant roots like carrot and parsnip from the grocer - they'll give cheap seed
plant Jerusalem Artichokes for an excellent crop of tubers, that are wonderfully crunchy scrubbed for salad - but only harvest them when you're ready for them, as they don't store well.
other tasty wild plants: ramsons (wild garlic), common sowthistle (lettuce-like)