White butterfly flying in the drainage dyke

I peered down into a very deep drainage ditch wondering what I might see. A kingfisher, possibly? No such luck. But this unidentified plant's mauve flowers were certainly very attractive to this white butterfly. It's wings look very ragged so possibly it is near the end of its life. Is it a cabbage white? As usual I am rather hopeless at indentifying insects of all types. But I did enjoy seeing it flitting between the flowers.