Hi, a lot depends upon how sheltered your vegetable plot is, or if you
want to use cloches to protect from frost. Spinach is good, spring
cabbage can be sown now and planted out Oct, Cauliflower, leeks and
japanese onions are other options. Also, consider sowing a green
manure on spare ground to over winter and dig in in the spring. Red
clover looks good whilst giving lots of humus. I'm trying vetches
(tares) this winter as well.

I've also heard of people growing new potatoes in containers ready for
christmas!