Until late August you can be sowing spring cabbage and spring onion. Until about 10th September, you can sow lambs lettuce and mizuna outdoors too.
For INDOOR salads (in greenhouses, under cloches, in boxes of compost/manure in the conservatory), the first half of September is good for raising plants such as lettuce, spinach, oriental leaves such as mustards, rocket, winter purslane, chervil and coriander.
But be careful with your lettuce - slugs are partial to that, more than the others I mention, except spinach.
All of these salads need the most care in their first two months after sowing. Once established they resist the cold and dark and go dormant for quite long periods until light levels improve in late February. These salads need protection from wind and rain - hence growing undercover - but do not mind frost, so fear not if they freeze. I grow them all in polytunnels where nights are often just as cold as outside, and the plants survive well. They are protected from the ‘weather’ but remember to give them plenty of ventilation, leaving doors open - more harm comes from high humidity than from gently circulating the air with these plants.