Sadly Ben any repairs on an 18 month old iPhone will be fully chargeable.
Get a detailed estimate before proceeding. Apple will often simply offer a discounted replacement.
Apple may claim that you can return the phone to giffgaff for a 'free' repair.
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 are constantly trying to hide behind 'non existent' legislation (or their 'interperetation' of it) when trying to justify only offering a 12 month warranty.
Sorry, but any repair, save for you being able to provide the stated evidence, will be fully chargeable.
*** You would need to get, as 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.
Apple place a three-year life expectancy on an iPhone before replacement so the absolute maximum contribution towards a repair on, as example, an 18 month old phone, would be 50% 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.
I wish I could give you a more positive reply.