Hi James,
Yes, this is working as designed. Severity 1 cases cannot be raised directly through your online account, however you have two options to do so.
1. You can raise the case through your support account and then call Customer Care to raise the severity of the case to a severity 1
2. Call Customer Care directly and we will raise the severity 1 case on your behalf.
Hope this answers your question.
Thanks,
Elke