There are some veg that you grow through the summer and then harvest throughout the winter, like Brussels Sprouts, winter cabbages such as Savoy and January King, and Kale. Summer sowings of chard will stand most winters and provide you with pickings in mild spells and in early spring. Also you should sow Broadbeans Aquadulce Claudia in the autumn (I do it in Oct/Nov) and then you will have a crop from them about May time.
This article will give you lots of information about growing winter veg and salads http://www.sarahraven.com/how-to/growing-food/9/winter-salads-garden
You can also dig part of your allotment over and leave it fallow for the winter, and the frosty weather will break down clods of earth, or you can sow a 'green manure' crop in the autumn and dig it into the soil in the spring. This is very good for the soil - this article explains about it.