Complex sentence: The captured frigates were valuable prizes but were not bought into the Royal Navy, thus denying their captors a significant amount of prize money: they been brought into port shortly before the Peace of Amiens and thus were deemed surplus to Navy requirements and not commissioned.
Split into simpler sentences: The captured frigates were valuable prizes but were not bought into the Royal Navy, thus denying their captors a significant amount of prize money. They returned to port shortly before the Peace of Amiens and thus were deemed surplus to Navy requirements and not commissioned.