On the subject of "Cabbage" white butterflies I thought I'd clarify matters.

I find it odd that someone hasn't contradicted Malcolm since he listed the
Green Veined White as a cabbage feeder. I have never seen it listed as such
in any literature. However I was once told by an expert that he had been
told by a MAFF expert that it was occasionally the case. Most butterfly
people would be suprised to hear this.
I don't know how accurate this is since butterflies do occasionally lay on
the wrong plants by accident particularly if they are related to the
foodplant but it doesn't mean the caterpillars survive.
Also the caterpillars of the Small and Green veined whites are very similar
in appearance.

The adults are similar and it is difficult to tell them apart with certainty
if they are flying more than a few feet away.
I have developed a rule of thim that usually works. That is if I am close to
habitation where cabbages might be found it is usually a Small White and If
I am not it is usually a Green Veined White. Not always accurate but more
usually right than wrong.

There are a number of White Butterflies in Britain

Wood white white - caterpillar feeds on Birds Foot trefoil and similar
plants
Green veined white - usually wild crucifers like javk by the hedge and
cuckoo flower. also Watercress.
Small white Cabbages, - nasturtiums and other crucifers
Large white -Ditto

Female Brimstone (male is Yellow female pale greenish white)- Buckthorns
Orange Tip (Female does not have orange tip) - Cuckoo Flower Jack by the
Hedge and a few similar others rarely.

There is a theory why these butterflies resemble each other.
Apparently the Large White is distateful to birds who quickly learn to avoid
it. THe others then gain protection from looking like it. This phenomenon
is well known in tropical butterlfies whi often have mimetic females and
non-mimetic males, which of course fits for the Orange Tip and Brimstone.

--
Neil Jones- http://www.butterflyguy.com/
"At some point I had to stand up and be counted. Who speaks for the
butterflies?" Andrew Lees - The quotation on his memorial at Crymlyn Bog
National Nature Reserve.