When did you plant them. The one shown looks to be firm. You can use it now. Hispi is a different cabbage in that you can grow it almost all year depending on when you sow the seeds or plant out.

Agree, now if you want to but it will probably heart up a bit more if you leave it.

I'm not sure when I planted them, possibly May. I bought them as plants from a garden centre.
Will they last a few months if I pick them as I need them, or will they be affected by frost?

You can pick them as you need them. We have lots of cabbage white butterflies going around still. If you put a mesh over you cabbages it will prevent the butterflies getting on to the cabbages and will afford some shelter from the frost.

Thank you. I've had enviro mesh on them since I planted them and it's kept the cabbage white at bay.

That is good. They look nice and clean.