These are the general practices nearest E20. All of them treat cats alongside other animals.
It looks well and keeps coming back
Probably owned and probably fine. A paper collar tag asking the owner to get in touch is the polite first step.
It looks thin, hurt or scared
Any vet will scan a chip for free, usually without an appointment.
You think it’s been abandoned
Celia Hammond in Canning Town take E20 cats. Don’t move a cat you’re unsure about.