In last year in June I bought iPhone 5-pay monthly. In last week my Phone turn off and I couldn't turn on him. Battery was low and I couldn't charge this battery. I went to O2 shop and lady checked this phone she tryed to turn it but she couldn't and she said so probably problem is with this button whitch we use to start phone. She sent my phone to repair service. Today I went to shop and ask about my phone and one men from staff checked and said so I need to pay 230 pounds because screen is broken and they need to change him. I want to ask how it is possible to my screen is now broken!!! I am very angry because I am without phone whitch I need to work, I need pay monthly for this phone one and half year yet I lost my minutes, messages. In my opinion this is not fair! I feel scammed.
If you search this forum for issues with o2's repair service you will find many such complaints of falsely reported issues.
I suggest you ask them to return the phone unrepaired and inspect it yourself - you also have the evidence of the shop assistant to support you that the screen was not broken when sent, so it's either not broken or damaged by o2.
If it comes back not broken and the phone is in warranty I suggest you take it to an Apple store to be fixed and also raise a complaint with o2 about their repair service.
Apple warranty check https://checkcoverage.apple.com/gb/en/