Cealstrina argiolus

The adults are most easy to identify when at rest, since their pale undersides are distinctive among the blues found in the British Isles, with the possible exception of the Small Blue, which is much scarcer (and, as its name implies, much smaller). The forewings of the female have broad black borders that are absent in the male. However, the adults only tend to open their wings in weak sunshine. Second brood females generally have broader black borders than first brood females.  A particular characteristic of this blue is that it will fly high off the ground, distinguishing it from other blues. In this respect, they are more similar in behaviour to a hairstreak.

Courtesy of Naturespot - www.naturespot.org.uk