There are still quite a few things to sow in August that will provide you with a good supply of food over the winter period. Late croppers such as Spring Cabbage and Winter Roscoff Cauliflower can be sown in modules or seed trays to plant outdoors in Autumn for a crop next spring / early summer. Cabbage can also be sown directly into drills outside. Plant out cauliflowers, as well as hardy kales.
If you have a bit of spare space in the polytunnel, spring onions, spinach, rocket, mustard and salad leaves can still be sown and it’s not too late for a last sowing of turnips and some fast growing radishes.