One of the most common butterflies: the Large White. 

I think the two spots on each wing indicate this is the female of the species. 

 Also known as Cabbage White, and they do indeed like a bit of brassica. Or rather the caterpillars do which is what does the damage. The Large White will lay up to a hundred eggs two or three times a year which results in a lot of hungry mouths to feed.

 These photos were taken on the allotment and I notice my neighbours have netted their cabbages to protect them.