I feed the tips of nettles to my shrimp and give them a quick blanch to help make them easier to eat. I've also tried chard but they didn't seen overly keep compared to the nettles. Dandelion leaves are a good source of nutrients and i blanch them as well. One of the best foods is nettle pollen and I collect and dry it to feed over winter. Other flowers like from runner beans are also good and you probably have more choice with an allotment.
For longer lasting food oak, beech and hornbeam leaves seem to be eaten plus common fruit leaves like Apple, pear, plum or cherry. Small pieces of these woods are also safe.
You can make a simple food to be frozen from a 50-50 mix of nettle and dandelion leaves mixed with a little Spirulina and gelatin.