A female orange-tip butterfly, Anthocharis cardamines (Lepidoptera: Pieridae). A male was flying above and kept coming down and mating with the female, and the female kept raising its abdomen towards the male. Unfortunately, the male had flown off by the time I got my camera out. (It is only the males that have orange tips to their wings.)
Photo taken in Whiteknights Park, Reading, UK, on 2014-05-03.