Bordered White.
(Bupalus piniaria)

With wings upright and that bright white flash across the hindwing and the interrupted white edging - should be easyu to identify.

Information:
Alas, there were 5 illustrations of the deceptively named 'Bordered White' - all seemingly as different as chalk from cheese. However, the diagnostic bit was the wings upright when resting, curved white streak across the under side of the hindwing (the displayed bit) with two interrupted downward curved dark Brown lines crossing the white streak. Oh, and the body - striped in muted colours.
They are quite common throughout most of Britain and can be seen flying May upto, sometimes as late as, August in pine plantations and woodlands. Their fore-wing length is 17-19mm. Males are somewhat different to females but obviously feathered antennae denote a male.