Allotment fruit and vegetables taste qualitatively different to, and have more vitamins, than the elderly items sold by supermarkets. One can only feel sorry for people who haven't enjoyed the flavour of produce picked and eaten the same day! The advantage of such freshness is strongest for salad and fruit since their flavours are not diminished by the cooking process. In our rather chilly climate, you can grow a great variety of salad vegetables, including tomatoes, cucumbers and peppers in a greenhouse, lettuce, mizuna, red cabbage, spring and mild onions, celery, baby turnips, peas, carrots, radishes, fennel leaves, which have an aniseed flavour, and young kale leaves. I will mention some slightly more unusual plants below. Most of these plants need to be netted, and protected from slugs and snails, particularly in damp weather.
Everyone has their own favourite varieties. I love the flavour of small black cherry tomatoes. You don't get a huge crop, but, then, supermarket tomatoes will be cheap by the time they are ready to eat. I like Magnum F1 cucumbers. These huge plants can be trained up to and across the ceiling of a greenhouse. The large cucumbers are best eaten halved or even whole very soon after being picked. They are crispy and delicious. I plant mild hardy onions, bought from the Newcastle Green Market, in the autumn. They are untroubled by frost, even in a cold winter such as our last one, and don't seem to suffer from blight. For lettuces, I commend the loose leaf varieties. They come in a variety of colours ranging from green to red, and, like hair, can be curly or straight. You can pull off a few leaves from each plant every day until they turn bitter or go to seed and need to be replaced by new ones. If you grow a season's plants in modules with a bit of overcrowding, they will wait patiently to be planted right through the season.
And now for some slightly less usual plants. The huge advantage of 'miners lettuce', otherwise called winter purslane or Claytonia is its hardiness. It was originally a weed which Alberta miners ate during winter as a source of vitamin C. It actually continues growing outside in our winter, although I protect mine with cloches. It has succulent mild leaves and produces flowers in the spring which can also be eaten. I swear that mine continued to grow during the unusually cold snaps of last winter!
I also commend red mustard. It is slightly spicy, but only in a mild sort of way, and also very hardy although it doesn't seem to actually grow during winter. Early autumn plantings will provide leaves through most of the winter and into the spring when it will start to regrow. The young leaves taste best, and it will need replacing when it starts to go to seed. Both Claytonia and red mustard can be grown throughout the year. They can also be over-wintered in the greenhouse when there is little else there, but will be fine under cloches or even unprotected outside.
And, finally, sorrel is actually a perennial plant which seems to produce leaves year after year. The main problem is to stop it from spreading. I grow mine in a box and do nothing except feed it occasionally and water it during dry weather. The leaves have a delicious lemony flavour. Like baby spinach, they contain oxalic acid, but a sprinkling in your daily salad will be perfectly safe to eat.