I'd hold off for a while yet as the soil will be very cold at present. Once the soil starts to feel warm to your hands then you can start sowing things. Seed planted in cold soil does not germinate but waits for the warmth.

Tell that to the weed seeds which are germinating in our tunnel at present! But, yes, it is a little early for most things. You could try some Spring onions if you are desperate to begin.

Also, rocket does better early and late in the season as it tends to bolt in the summer. The other germinations you can start at this time are pak choi, lambs lettuce, land cress (same as water cress but grows in the bed and is more peppery) and winter density lettuce. Winter spring onions as owdboggy suggests are great, as are wall flowers. I have also started my carrots at this time, to avoid carrot fly, though I plant them in 18" high pots to avoid this in late spring. Cover with a bit of see-through plastic or bubble wrap to keep the frost off. Germination make take a few days longer, but then you will have an early crop of something in March/April ... weather permitting! Winter raddish will still also sprout early, though best planted late summer/early winter for cropping late. Depending where you are and the weather ... have a go with any of the above!! Worcester, in the west midlands, has been good to me in recent years.

And I've just noticed Polytunnel is in Devon so much milder than a lot of the UK. So yes good suggestions Avkq.

Thanks for all your advice polytunnel