Yes you are right to be upset. You tried to put it right and they refuse to cover the costs. I do not know whether you can claim on your comprehensive insurance for that. Any damage in a newish car will show up easily and you want to make it right. But I do not know whether that will be value of money.
For the paint and the poo, maybe they used cheap paint, you have to check second generation cars like mine and find out how poor the paint is. You check them from a distance and they look great, you step closer and you ask yourself how that is possible. Every chip stone is different that the other and every poo will be different than anything else but yes they could have put some effort to make the paint a bit more robust to cover the majority of the expected damage.
I thought the 20 years for 3 cars meant a new car every 7 years or so. Mine is 9.5 years now and it is bad how the paint has turned out where it has been attacked, i.e the whole front part of the car, the areas hit by the water the wheels spray and the notorious roof on the second generation cars, but if £900 is what they ask for each panel, that is the part exchange value the dealer pays for these old cars. So it is not worthy. I think gradually the third generation cars will age better than the second generation cars because there is a difference in quality between the two. So in that sense, I would worry that your car will get really bad but it will start showing old sooner or later. The less you drive it, the better it will look though.
Edit to add:
Despite what the damage is, I would not like to respray any part of the car, especially if it is a whole panel. Maybe you could try to get some advice from quick fix easy repairs shops that do not respray things and find out whether that paint correction can happen in that way. Maybe professionals in cleaning/detailing they will know more. But a 18 months resprayed car may look worse than the current 3 5p coin bits not being perfect.