Chard does well even in the wilds of North Britain. Salad leaves (lettuce, rocket - actually spinach and chard will do too) are easy to grow, good to eat and will save you money. Herbs are always worth growing. You can (almost) never have too much parsley or chives.
Some things that look and/or smell nice too are worth it - I recommend lavender, while pot marigolds (bseides looking nice) can be a useful sacrificial plant for slugs and snails.
With small borders like those, you can mix plants up quite a bit - which can look better than strictly ordered rows (and may help control pests).