There is no single qualification that employers require. Some providers offer Ofqual-regulated awards and university-level certificates, and these can help early in a career or where an employer wants a certificate for its records. In fifteen years as an EA and in hiring PAs and EAs for my own team, I never asked for one. What I looked for was evidence of judgement and the leaders a person had supported. Choose training for what it changes in your work, not for the certificate.