Why do they need express permission from BMW? The most they need is something from your insurance company to say that you've declared this change to them but even that is your responsibility.
As for voiding warranty. While the factory warranty won't cover the aftermarket tyre for other items they'd have to prove that the changing of the tyres actually caused the failure.
Personally I'd go to my local trusted garage.
As for voiding warranty. While the factory warranty won't cover the aftermarket tyre for other items they'd have to prove that the changing of the tyres actually caused the failure.
Personally I'd go to my local trusted garage.