lovely picture! This is an Orange Tip, Anthocharis cardamines, butterfly. From the tiny hint of orange just visible it is probably a male. The specific name of cardamines refers to the fact that the food plant for the caterpillars is Cardamine pratensis - a common plant of damp grassland, amongst other things.
I'm still waiting to see my first one for this year...
...One of my favourite butterflies, and always a special joy to see the first one each spring.
Growing up in the UK, one gets used to our particular butterflies, and the fact that some are quite distinct - such as small copper, purple emperor, and orange-tip.
I have lived abroad twice (currently in Cyprus), and that does change one's perspective - reminding one that the butterflies that occur in the UK are but a sample of the wider range of species.
For instance, over here we have the cleopatra, essentially a brimstone with orange patches on the yellow wings - dazzling. The only orange-tip that occurs here is the same as the UK one, and still delights me. But further afield, there are other orange-tips, such as the Moroccan orange-tip - http://lepidoptera.pro/taxonomy/8821
I am primarily a bird watcher but enjoy all wildlife. We were in Spain in March and fortunate enough to see a Moroccan orange tip. We are going again in the winter and looking forward to seeing many new species.