Cuckoo-spit, Spittlebugs and Froghoppers (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.