Hi Dale,
English makers, like all others, mixed and matched largely owing to supply, customer demand, and use. Silver is softer than steel, iron, brass or bronze so it would make inferior buttplates and triggerguards with respect to wear. Certainly there were many presentation guns tricked out with silver but I doubt many were meant to be used very much. So many makers made the hardware that had to be tough out of iron or steel, but used silver for the decorative parts. Silver shows up a lot on pistols but they generally were not used as hard as long guns. Also, during the 17th and early 18th centuries, steel was very expensive because it was always in short supply. Methods to make large quantities had not yet been invented. In the 17th century, steel was often considered more prestigious than silver because it was almost as expensive, wore much better, and could be polished bright. In fact, it often keeps a bright silver color longer than silver before tarnishing. On firearms, steel was in demand.
dave