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For those who haven't encountered this before, 'cuckoo-spit' is created by the nymphs (early stages) of some froghopper bugs. They suck sap from plants, and then excrete it in 'frothed-up' form to provide themselves with a sticky, frothy covering that provides protection for the soft-bodied insect hiding inside.
As far as I know it's called 'cuckoo' spit simply because it tends to start appearing at the same time as cuckoos are heard (or at least did, in the days when cuckoos could still be heard easily).
For more see:
Entomologist and biological recorder
British Bugs has cuckoo spit froghoppers as Aphrophoridae. Cecropidae with underground nymphs in UK.
Philaenus spumarius is probable though not certain
Lat/Lng: 51.282972552127, -0.62163412570953
OS grid ref: SU962546