I could be wrong, but I would guess that your quandary isn't to do with the policy as such, but the way it's been gamed by car manufacturers. That diesel would likely have a much higher road tax if its actual emissions were taken in to account, and not just the lab-based ones.

Get a petrol car pay the tax would be my recommendation. Depends on what you need too. My 5-year-old petrol Fiesta was <£10k and is £0 road tax.