iMessage is just data so there is no way for o2 to distinguish between an iMessage or internet use, for example.
If you have iCloud back up turned on your iMessages should be backed up on there so if you were to restore a backup you may find your missing messages.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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