There are quite a few species of blue butterflies in Britain, but if you ever see a small blue butterfly in a tree or bush like this one, then it's almost certainly the Holly Blue Butterfly.
It is our only native blue butterfly more commonly found in trees and bushes (rather than flying low over grassland), feeding off ivy in the autumn for example (like this one). Is that the first time I've mentioned Autumn on "Blue-Grey"?!
If you are in any doubt though, look at its underside. It is a silvery blue with a few elongated black dots, (not ringed with white and NO orange). Completely different to all of our other blue butterfly species.
I found this one in my mother's back garden, in High Wycombe. A very beautiful butterfly... check out the stripes on the legs and antennae!