Prunus avium 'Plena'
Prunus avium 'Plena'. Photos taken 15th April 2014 (a very early year), when a lot of the flowers are opening but the leaves are still small and reddish.
This a double variety of Wild Cherry. It flowers a bit later than most single-flowered Wild Cherry trees, and even from a distance has a noticeably different appearance when flowering: more tufty and irregular, the flowers less obviously sleeving the shoots.
When you get a bit closer, it's a magical tree, especially when the skies are like this. The pure white blossom has a touch of the same frilly opulence as 'Shirotae', but here it's sprinkled through a rangy, open canopy.
[For comparison, the photo below is a single-flowered P. avium, showing the blossom sleeving the shoot: ]