A PGDL is not a degree. It's a Diploma. Although people talk of "converting" a degree in a subject other than Law into a Law degree by obtaining a PGDL, that isn't what happens. The first degree remains a degree in whatever it is. The classification of that degree is not modified by success or failure in the PGDL. The PGDL is essentially a pass/fail thing. I wasn't aware until I read your post that any of the PGDL providers use degree-style classifications to indicate performance in a GDL. I was aware that some use terms such as Merit and Distinction.
In my experience of assessing candidates for pupillage and tenancy in chambers, and candidates for employment in law firms, it does not matter much or at all where a candidate obtains a PGDL or how well the candidate does in the PGDL, so long as he or she passes. Performance in the undergraduate degree is more important. Doing well in the PGDL is a bonus, but isn't usually a decisive factor.