In the context of the roofing material I am using, at what minimum degree / pitch would the SolarFlash be recommended?

Per building regulations and the manufacturers specification for that roofing material, but never less than 20 degrees.

SolarFlash is designed to give a maximum 4 inch / 100mm headlap, and is designed for single lap tile layout and double lap slate layout (with slates, the pitch is as per the building inspectors recommendation).