S&P sets the sovereign rating of almost each country in the world, and a country that has no rating can hardly tap international finance markets. France, UK, the US, like most OECD countries, are AAA rated, which is the highest rating on a AAA to D scale.
If the rating of a country is downgraded by even one notch (for France it would be AAA-, this immediately triggers investor suspicion, money fleeing the country and increases cost of government debt raising.