Thank you for your responses guys! I will give CS a call then.

I wonder why though, each time I try to unlock through the My O2 app, they send me this email:-

Dear Daniel xxxxxxx

Thank you for your unlatching request for IMEI 355xxxxxxxxx. The
mobile number that you provided is 07xxxxxxxx.

I'm sorry, we have not been able to unlatch your device at this time.
The reason given is:

The handset credit agreement must be paid off before we can unlatch your
iPhone.

Thank you,

O2 Customer Service