I wouldn't worry; forgetting to sign is just one of many administrative mishaps that occur every single year.
Your grade would not be impacted by this and the fact that it has not been brought to your attention by staff shows it's not that big of a deal, although it make feel like so.
People have made errors that would make identification hard, e.g. wrong candidate/centre numbers, but as long as your information was correct, they'll know how to contact you if they need to.
As you said, it's all checked before being sent off, so try not to worry.