You'll struggle to reject it based on the circumstances. Unless you can prove theirs something wrong with the chasis. I think the best thing you can do mate is let them fix it and then sell it when the day comes. Sounds like you've just been very unlucky, it happens unfortunately. It will be fixed more than likely before you need to sell it. If not that's grounds for a rejection. But let them know your intent now before it's too late, both your local dealer and Audi customer services.
It isn't my responsibility to prove the chassis is at fault, it is Audi's job to prove to me that it isn't. If they repair it, I'm not prepared to sell the car privately because when it goes wrong I don't want it coming back on me! I could lose thousands by hanging to trade it in.
The dealer, Audi UK and finance company are all fully informed that I want it rejected.