Why this is not a dropdown
Getting the regime wrong is not a formatting problem. A fixed-term AST issued for a tenancy that the Act says is periodic is the wrong document, and you find out at the point you need it to work.
So the builder reads the dates and produces the agreement that matches, rather than trusting anyone to remember which side of the cutover a tenancy falls on.
What comes with it
The agreement is built alongside the written statement of terms, which has to be given for a tenancy entered into after the cutover. Both go out through the same send-to-tenant route, so the record of what was served sits in one place.
Signing
Send it for e-signature and each signature is captured with the time and the signing details recorded against it. You get the signed PDF; the tenant gets their copy through the portal.