Tim
The caterpillar on your cabbage is the larvae of the Cabbage Moth, now the Cabbage White butterfly is something different again. (although the aim in life for all is to grow eating your food) In fact there are two cabbage white butterflys, the Large White and the Small White.. Large White larvae, small white larvae and the cabbage moth larvae are all on this link.
http://www.hdc.org.uk/herbs/page.asp?id=8
Dave