Although we here, in the western U.S., dont have a climate condusive to late season cabbages, the same tips would apply as to the early season varieties.
First, mulch the base of the plants with several layers of newspaper. Just put the sheets down around the plant and spread a little soil to keep in place. Then soak the newsprint thoroughly with water and it will stay in place. Secondly, keep a sharp eye out for various and sundry butterflies, or so they appear. Especailly the Cabbage White, which I'm sure you must have in the U.K. They're smaller that the more decorated varieties and they are there for one purpose... to lay eggs that will hatch into cabbage eating little worms. I use a chemical dust to protect the cabbages. Don't put it on except early morning or late evening and never when wet. Any garden store will provide a good brand. Re-appy after rain or if you water with a sprinkler.
If you are truly adverse to applying chemicals, mix up a solution of 2/3 buttermilk and 1/3 water in a spray bottle and spray the plants thoroughly. While the mixture is still wet, pour or spray on a light dusting of any household red pepper. You'll have to re-apply after any rain, but it does work as well as keeping slugs at bay. Some people use amixture of garlic juice and water... I haven't tried that as yet.
Thirdly, once the cabbage begins to form into a head, tie up the larger outer leaves to protect the cabbage from sun burn... Best of luck!