Marbled White Spot.
(Protodeltote pygarga)

Resting in a typical pose - looking like what it is, a Marbled White Spot.

Information:
The white spot is that pair of white areas that make a sort of sharply delineated crescent where the two trailing edges of the forewing meet - the most obvious feature of the moth. The grey and brown marbled background varies only slightly in colour.
Although they can often be seen at rest on fences and tree trunks during the day, at night they can be seen flying May-July in most parts of Britain where there are woodlands, grassy areas or moorland.