I would send a letter of complaint to John Lewis saying you were not impressed with being accused of intentionally damaging the faulty product they sold you and highlight how common this exact issue is by pointing them at any number of these images online...
Just after posting on here I actually sent John Lewis an email with the exact points you said. I included a link to the apple charger on their own website were people have left negative reviews due to having the same cable fraying issue.
I would send a letter of complaint to John Lewis saying you were not impressed with being accused of intentionally damaging the faulty product they sold you and highlight how common this exact issue is by pointing them at any number of these images online...
https://www.google.co.uk/search?q=macbook+charger+worn+out&safe=off&espv=2&biw=1436&bih=780&source=lnms&tbm=isch&sa=X&ved=0ahUKEwion6KOhN3PAhXO0RoKHRZ6BLoQ_AUIBygC&dpr=2