Confidence: I'm as sure as I can be.
Notes: Cercopidae are only represented by Cercopis vulnerata in the UK, which has underground nymphs. The rest, including the Common Froghopper, are in the Aphrophoridae. The nymphs are hard to tell apart, but if it was on low vegetation then it's probably Philaenus spumarius. If on grass or trees, then there are other options.