You can probably use regular green cabbage or savoy cabbage instead - it'll take a little longer to cook, but is a pretty close substitute.

P.S. Given the choice between cabbage and savoy cabbage, I'd go with the Savoy it you can get it.

Many thanks, I can certainly get hold of Savoy cabbage here so will give it a go!

I think it's called "Chinese" cabbage in the UK, if I remember correctly. Baby bok choy would work, as would escarole.

Savoy cabbage would work well, but so would plain, everyday cabbage. If you use regular cabbage, slice it much more finely than that in the recipe's photo. You could also use dandelion greens, escarole, frisee, or even romaine. My grandmother made a similar dressing that she used on lettuce as well as cabbage.

You could also use the Dutch pointed cabbage. It has a very distinct pointed edge on one end and the leaves are less compact than traditional cabbage.

Look for "Chinese leaf" or "Chinese cabbage". It's also called "Hakusai" in Japanese.