2. Under UK consumer law Apple would have to prove that jailbreaking the device directly contributed to the reason for warranty claim.
Are you sure about that ?
I would argue that if Apple can demonstrate the device has been jailbroken, then they would be within their rights to make a case that the device has not been used in the manner for which it has been designed.
If you've altered the goods, which jailbreaking most certainly is, then you're deemed to have accepted them.