Green’s Dictionary of Slang

(UK juv.) saliva, phlegm, in the context of spitting.

1998		B. Robinson Peculiar Memories of Thomas Penman 64: Before you can flob, you gotta suck in, you can only get flob from your lungs, and you can’t get real flob unless you smoke properly.
2001		OnLine Dict. of Playground Sl. 🌐 flob n. saliva.