Using your phone to get your managers number would not typically be considered a data protection breach. UK GDPR are primarily concerned with unauthorised or accidental disclosure of personal data. In your case, wifey informed your manager about your emergency which seems reasonable and necessary. The alternative being work would have no clue where you are?!
As for ACAS, they are great for advice and will represent you, however, the company can decline to engage with them, this would result in ACAS issuing a certificate and you having to go to tribunal - expensive and time consuming and constructive dismissal/wrongful termination is difficult to prove/win - although check your home insurance for legal protection for employment disputes. Are you in a Union?
I think the AWOL is a stretch, you were taken to hospital by ambulance, what if you were in a car accident and unconscious?! Would work still write you up for AWOL for not calling in? As for the GDPR - another stretch.
It does seem that work is trying to manage you out, but it also seems like they are grasping at straws.