What can I plant in pots for the winter?

[Does the Cooking]

Is to too late to plant anything for the winter?

When should I be thinking about seed potatos?

[Flummery]

I wouldn't think about planting seed potaotes till the spring. If you have a greenhouse you could get some in really early - Jan/Feb but outside I'd wait till mid-March at least.
You can plant some autumn onion sets or some garlic now in pots.

[Cutecumber]

You can order your seed potatoes now - they will be delivered early in the new year and you plant them in the ground when you can be sure the tops won't get frosted (end March, early April, for example). They are tender and will not stand a frost.

At this time of year, most plants are shutting down rather than starting their growth - there are, however, a few things you can get going which will crop for you in the spring...

garlic - put some cloves in compost now and they will get going well.
broad beans - sow now and you should get an early crop in the spring - but you need to choose the right type.
over-wintering peas - more of a gamble, but should produce an early crop if you can give them a little protection. Again, look for a variety that says it is suitable for over-wintering.
onions - some onions can be planted now

If you have a sheltered space, you could sow some salad leaves - the oriental mixes are probably the best for this time of year. They won't grow as quickly, but you should get a crop of tender leaves.

[Madasafish]

Rosetta lettuce in a cold greenhouse.
I have 20 in pots.. of varying sizes...