What is millboard or fire rate board ?
Fire rated board is required when the edge of the gas burner is within 140mm of the glass splashback. This is a Gas Safe Law.

What is the difference between Starphire glass and Clear glass?
Starphire glass is the most optically transparent glass available; which means your colour choice for your splashback has the least possible variation to the colour chosen. While Clear glass has a high concentration of iron; so when the glass is painted it gives off a green tinge.