I use mine to over winter plants that are a bit tender, if its a really bad winter they still freeze and die but last year osteospermum and pelargoniums flowered nearlythe whole of the winter and got such a good start in the spring

Monty Don says sweet peas, new potatoes (for harvesting on Xmas day) and winter salads. Might be worth thinking about.....

If you sow salads make sure that you buy winter varieties. Try winter lettuces. They grow quite slowly but are welcome fresh leaves in winter. Keep the greenhouse ventilated on mild days to discourage rotting.

Kale, mustards, bunching onions, spinach, and radishes also do well with light frosts.