Apple iPhones only have a 12 month warranty, there is NO EU law that mandates a 24 month warranty so once again, absolute rubbish from Apple.
Out of warranty is out of warranty I'm afraid.
Under both UK and EU consumer law you have to provide conclusive evidence*** that the fault was either inherent to the design of the phone or existed during the initial warranty period.
Apple is constantly trying to hide behind 'non-existent' legislation (or their 'interpretation' of it) when trying to justify only offering a 12-month warranty.
Under UK consumer law on NEW products, the full onus of responsibility falls on the owner after six months from delivery to prove that the fault was either inherent to the design of the phone or existed during the initial warranty period. On second-hand products, this onus begins immediately.
Sorry, but any repair, save for you being able to provide the stated evidence, will be fully chargeable.
*** You would need to get, as an example, an independent engineer's report at your own expense which provides conclusive proof that the fault was either inherent to the design of the phone (they won't find this - trust me) or pre-existed the expiry of the warranty (almost impossible to prove).
With this, you then have the basis of a case for a claim against the retailer (in this case giffgaff).
Even then any reduction in the repair cost will be proportionate to the age of the product, simply having the evidence does not 'extend' the warranty, and will be based on the expected life of the product if fault free.
UK and EU courts place a 'reasonable expectation of durability' on products when determining repair costs.
Apple places a three-year life expectancy on an iPhone before replacement so the absolute maximum contribution towards a repair on, as an example, an 14-month old phone, would be c.70% even when you have provided the, almost impossible to obtain, evidence.
My simple advice would be, in future, to buy a phone with an industry-standard 24 month warranty.