Have noticed this for many years and never known what it is!!!!
No interactions present.
Cuckoo spit is produced by several species of froghoppers, but not Cercopis vulnerata, which feeds on roots underground. Most of the other species feed on grasses or specific trees like willows... which means this is almost certainly the extremely common Philaenus spumarius, which is about the only one that feeds on any low plants it can get its rostrum into. :-)
Lat/Lng: 50.6007, -1.2199
OS grid ref: SZ553781