Scalloped Hazel (Odontopera bidentata)
Not just a drab brown but variable brown areas and with a darker brown band through the middle bordered by silver threads and in each of these bands a darker eye. It looks as though it is wearing a mask. They also have this 'scalloped' edge to the hind wing which I suspect helps to break up its outline and aid its camouflage.
Flying in May and June you can find this insect at rest on walls and fences but they are so hard to spot because of that colouring. They also come to light so you can find them fluttering against your window.
They are not uncommon in England and inhabit parks and gardens as well as open woodland areas.