So if Russia does not nuke London and Paris, but nukes the others, the Anglo-French response should be...

My purpose is that people should stop thinking every nuclear war will be a life-extinguishing global extinction event. Such that if Russia or China use tactical nuclear weapons, we should not fear a response in kind. If they and other nuclear states ever get that there is almost nothing save an impending attack on our countries to warrant a nuclear response, they are more likely to step out of line.