I can’t offer the "correct" answer but will share what I did during the restoration of my Tiger. I painted the undercarriage in satin black using an enamel paint with a rust inhibitor.
Opinions vary about under-coating but I was swayed by the logic of not using it. For me, I don’t plan on driving the Tiger during winter months or in conditions where it will stay muddy or dirty for very long. Therefore, a good coat of paint on the bottom should last long enough. If rust does begin to form, it will at least be evident on eye-inspection, whereas, the undercoating can potentially hide it for some time.