Any non standard wheels ( or almost any non standard anything) will cause problems with insurance in the U.K. They won't miss any opportunity to dismiss a claim.
As far as the warranty is concerned, it won't affect the whole warranty, as I understand it, only the bits it concerned, so presumably in the case of aftermarket wheels,, you would be stuffed warranty wise if the wheel bearings went. Which they would be quite likely to if you put on wheels with more than the designed offset. But I would not be surprised if anything went wrong with the suspension or brakes that they would take the opportunity to dismiss a warranty claim if a/market wheels are fitted.
Personally I think it is just too much hassle to warrant the cost. I'll stick with what I've got!