Yes, it's "cuckoo-spit" which is a sort of protective barrier created by frog-hoppers which are aphid relatives. They produce the froth by eating and then excreting and bubbling-up the plant's sap. It's function is to protect the insect so that they can feed on the plant sap without being eaten by their predators.

And they are not baddies!

No? They cause scattered die back of Rosemary twigs here in the desert.

From the Natural England website:

The common froghopper nymph or spittle-bug feeds on sap from plant shoots. Leafhoppers do little damage to plants in Britain. Most small birds may take these species.

So, not particularly wanted, but no need to get in a lather? ; )

There are worse...

"So, not particularly wanted, but no need to get in a lather? ; )"

I see what you've done there..... :-)

I was quite fascinated by these things when I was a kid actually - one of those improbable but fascinating bits of nature that you can find in your garden.