We have an allotment and we do try and grow some stuff through the winter, and we do try to get some variety.

Try it. It isn't nearly as easy as you think. Not all root veg are frost/snow hardy (hence the existence of clamps). Things grow so slowly in the winter that you simply run out before spring arrives.

It's known as the 'hungry gap'.

Winter greens are fine for vitamins, but not much good for calories.