From Apple’s website on
UK statutory rights:
Any defect or non-conformity of goods with the contract which becomes apparent within 6 months of delivery are presumed to have existed at the time of delivery. After the expiry of this 6-month period, the burden to prove that the defect or non-conformity of goods with the contract existed on delivery generally shifts to the consumer.
and
If the product does not conform with the contract after one repair or replacement a consumer is also entitled to exercise a final right of rejection and withdraw from the contract by returning the goods in exchange for a refund. In these circumstances, Apple may deduct a reasonable sum in connection with the period of use that the consumer had of the goods.
As you pointed out, it’s unsure how this would play out in reality; especially if the device was bought from a smaller 3rd party seller. For example what is a “reasonable sum?” Number of months in use divided by 72 (60 in Scotland?) What does “generally shift” mean in a court? I would think a common defect such as failing keyboards would still be on the seller, but who knows what a court might decide.
Not trying to be argumentative, just pointing out it is more complicated than bring it back and get your money back.