Brimstone Moth, Opisthograptis luteolata
According to http://ukmoths.org.uk it is a common and widespread species in Britain, occurring in three overlapping generations in the south, graduating to just one brood in the far north.
Adults can be expected on the wing at any time from April to October, depending on the locality and number of generations, and occurs in any suitable habitat.
The larvae feed on a range of trees and bushes, including hawthorn (Crataegus) and blackthorn (Prunus spinosa). Source; http://ukmoths.org.uk/show.php?bf=1906