If lampholders are designed for 12 volt operation (or any extra-low voltage), then no earth is needed. Protection is given by "electrical separation" (transformer).
The trouble is, when used with a 240v mains supply, then there is a potential risk of the casing becoming a "live part" if there's a fault. There is a Regulation that allows "live parts" if they're out of reach, but I doubt if that would apply here.
This is often a difficult situation when electricians try to do their best for the customer, but have to consider safety. In your case, I think most would either drill the casing to connect an earth, or insist on new holders with earth connections provided.
I take OG's point completely, but sometimes you're between a rock and the proverbial hard place.