I initially thought that this was a small white butterfly Pieris rapae, but in the comments, WilliamB pointed out that it is actually a green-veined white, Pieris napi (also Lepidoptera: Pieridae). On the underside of the wings of the green-veined white, the dark scales form bands along the lines of the veins, whereas in the small white they are much more evenly distributed. The flower in the above image is self-heal, Prunella vulgaris.
This is what the green-veined white looks like from above:
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, that of the large white on 2009-07-16, those of the green-veined white on 2009-07-25.