Definitely plant spring cabbage as these can be planted very close together, then you can harvest spring greens by thinning them in spring, leaving the others to heart up in April and May.
Also sow or plant some Chinese greens like Pak Choi as these do well in shortening days and are fairly hardy.
If you can get plants, many of the hardy curly kales are excellent crops which send up dozens of small fresh shoots in spring which will eventually produce flower buds and can be eaten like broccoli. They can be cut over and over again if well fed in the spring and given enough space to make big plants.
If you can get some plants of American Cress, this is a good winter salad, as is 'Mache' or corn salad which will grow slowly through the winter months.
A small clump of autumn sown broad beans like Aquadulce would also be worth sowing for an early crop in late spring, even though you may only get a taste.

I agree with Bertie and perhaps this planner might help you in the longer term;

http://www.thegardenersalmanac.co.uk/Planners/Preamble.htm