Anthocharis cardamines Orange Tip
Orange tips visit our garden every Spring, sometimes very early. Their food plants include Alliaria petiolata (Jack-by-the Hedge) and Cardamine pratensis (Cuckoo flower) which are both April flowers. On the LHS photo is another Cardamine which they seem to love: Cardamine rapahnifolia (Greater Cuckoo Flower). There were about 20 Orange Tip butterflies moving very quickly between the flowers here.
The cold weather stopped the butterfly in the RHS photo from flying and it was still sitting shivering on the brassica when I left.