A greenhouse will open up the possibility of growing many new varieties of fruit and vegetables to maturity, particularly in the North of the UK. Many of the semi-tropical plants such as tomatoes, peppers, melons, cucumbers, will crop well.
The greenhouse does not need to be anything expensive. On allotments, it is common to see home-made greenhouses stuffed to the rafters with semi-tropical plants.
Often people do not fully utilise their greenhouse during the late Autumn, Winter and Spring, when the protected space could be put to good use. In the Autumn, you could dry off garlic, onions, shallots, and Winter Squash. You can over-winter semi-hardy plants such as Bay, or tender salad vegetables such as mixed salad leaves. I always start off a few 1st early potato tubers in boxes of old compost in January, in the hope that I will be able to harvest a few potatoes in May. When you need the space for your Summer crops, move the potato box into a sheltered position outdoors and cover with some plastic protection.