Lots of things can be sown in September as the soil is still very warm and seeds germinate easily. The kinds of things you may want to grow are :
1. Spring greens. May need some winter protection in a cold winter, but plant closely together for fresh greens in the spring and early summer.
3. Chinese greens. This is the ideal time for them as they tend to bolt in summer. My favourites are:
a. Cut and come again Mizuna or Mibuna greens.
b. Pak Choi.
c. Kailaan Chinese Kale... produces a single flowering shoot on a chunky stem.
d. Any good Chinese cabbage. These produce a nice dense heart which is ideal stir fried.
If you can protect the Chinese greens from the worst of the frost, with fleece, you can crop them well into winter.
e. Black radish or Japanese radish, which make a large root which you cook like a turnip.
4. Turnips... grow a quick maturing variety like the long rooted 'Long de Croissy' or the round 'Tokyo Cross'.
5. Swiss Chard. The young plants should grow enough to survive the winter, with a little protection, to give you lots of spinach greens in the spring when they grow away.
5. American or Land Cress. Again, you probably won't be able to crop until the spring.
With all of these, it's best to sow in early September so you have enough growing time to get the plants established.