As colder weather approaches, you may find your cabbages nearing maturity (if they haven’t already done so.)
You’ll know when your cabbages are ready to harvest because they will have formed a firm head in the centre of the plant. Hopefully, this will grow to be a reasonable size, a bit smaller than a football but definitely bigger than a tennis ball. You may see some of the outer leaves beginning to grow away from the cabbage head and turn yellow/brown. This is usually a good sign that the cabbage has reached maturity.
Depending on when this happens, you may or may not need to harvest quickly. If the plant reaches maturity in the next week or so, while we still have warm weather then you may not want to risk the cabbage head from splitting (which it can do if it over-matures and draws up too much water/nutrients too quickly.) If the cabbage does start to split (it looks as if something has hacked into the cabbage head and ripped it) then just pull the cabbage up and cut off the good parts so that you can cook what you can.
If the plant is only just beginning to form a head now, and reaches maturity in November, for example, then once the weather is colder there is no harm in leaving the plant to stand in the ground as it won’t do any active growing once temperatures drop to around 5c and daylight hours are that much shorter. They will withstand frost well, and actually many gardeners find that the flavour changes for the better after a few frosts.