(Philaenus, Neophilaenus, Aphrophora spp.)
The cuckoo-spit that we see on plants in spring is caused by a Spittlebug, which is the nymph stage of a Froghopper. There are 10 species of Froghopper in the UK.
The Spittle-bugs feed on the sap in the plants and excrete the spittle that surrounds and protects them. They can be gently removed from the spittle for a closer look without doing them any harm, but they should be returned to the plant quite quickly.
This one is starting to excrete more spittle to cover itself back up.
After around a month as a nymph, they moult into an adult Froghopper that can fly and hop. They vary in colour, are about 6mm in size, and continue to feed on sap.
Most Froghoppers are brown, but these more striking ones are Red-and-Black Froghoppers (Cercopis vulnerata). They're common and widespread in England. Unlike most Froghoppers, the nymphs of these ones are rarely seen as they feed on roots underground.