what,if any lettuce will grow in a unheated greenhouse from now ?
Ideally it's better to start winter salad etc. off earlier (August/September) so they get plenty of growth in the longer/warmer days, then you can harvest them through the winter. I haven't been very organised about that this year so might try and start some now as well. Pak Choi and similar Asian greens are a good choice for winter salad.
I get the misticanza or saladisi mixed leaves. Scatter the seed on a tray of compost and cut leaves as and when necessary. Its not a hearted lettuce but a lot better than the bags of salad leaves you buy.