I'm not sure it's worth getting the birth certificates for the twins Eric and Dorothy, as you already have most of the information that's going to be provided on them from the baptism and Eric's death. The only additional information you'd get is the detail of the marriage and its previous issue, which you already know too.
Save yourself some $$$

The priest at the baptism or his clerk has entered the wrong name for the father at the twins. The BDM index confirms they are children of Thomas & Mary.