Yes, quite. They used whatever was most convenient or suitable. Most early stations were built of stone, though they might not have been GWR designed. But if there wasn't stone easily available, then brick might have been used. When Minehead station was extended in 1923, the GWR duplicated the original Minehead Railway Company design and used the same stone. Interestingly, GWR signalboxes were almost always brick or timber. Apart from an odd few, I can't think of any stone GWR signalboxes.