The yellow-barred longhorn moth (Nemophora degeerella) is less than a centimetre long and yet its antennae are twice as long in females and four times as long in males. Not surprising, then, that this and its cousins are generally called longhorn moths even though they do not have horns of course! From the antennae I would suggest that this photograph is of a female which is also slightly darker in colour than the male.
To see these in the sunshine sitting on a leaf like this is not uncommon as they are distributed across the country in areas of deciduous or mixed woodland. Most frequently, however, you see them in woodland clearings on sunny days in May and June dancing up and down (not dissimilar to a mayfly). They are much harder to identify and separate from their related species then as you cannot see their lovely and distinctive colouring. With the sun on them they are wonderful combination of gold on a metallic green back ground.