To the above suggestions could be spinach beet(perpetual spinach. Last year I lifted some established swiss and ruby chard into 10"pots before the first cutting frost,, and moved these in to the tunnel. They didnt grow much but sat in a state of suspended torpor but remained fresh and edible. New leaf appeared in late Jan and slowly advanced for about 3 pickings before Easter
I found that the major limitation is the dramatic fall in temperature in the early evenings, little retention of daytime solar gains and the slow recovery next day in the oh too often dull days of winter.
The advantage of a polytunnel is probably about 4deg C in air temp during the winter with reduced windchill and no wind burn/dessication, so things may not grow much but they survive for an earlier start when the days extend. Its light levels and day length that you need as well as air temperature, so its worth getting the outside clean and the inside free of green algae. Unless you have light and heat inputs(unlikely on an allotment), you cant achieve much more. However, this year I,m to experiment with a "hot-bed"in my tunnel just to see what grandad could do. Wiill report if sucesssfull . ADVICE WELCOMED.
