The only recourse for warranty providers that dodge claims is the Financial Ombudsman Service.
It takes ages for them to look at and decide, even then it is usually an assessor, rather than an actual Ombudsman, that decides unless you reject the assessor's opinion and appeal.
Best move is to avoid the hassle and not to buy with the more troublesome warranty providers.
...would you say that you definitely wouldn't buy something like this if I told you everything else was great?
How do you know everything else is "great"? Did you inspect every stage before it was covered up?
Are the joists undamaged and at the right spacings? DPCs? All insulation installed? No bricks or obstructions in the drainage?
Chances are if it looks this bad now there will be other, perhaps serious issues.
In answer to your question, I wouldn't buy a new home from a housebuilders unless each stage was signed off by an independent inspector I appointed. No plc housebuilder would agree to that and we all know why!