Why the Brave-class fast attack craft remained few

Why were so few Brave-class fast attack craft produced?

Because the Royal Navy had not settled what role it wanted them to perform and therefore did not require a large class.

Limited production reflected demand rather than a failure to build the craft. The Royal Navy did not want many because it was still working out how it would use them. That uncertainty limited the case for a larger production run. An unsettled operational concept can constrain procurement even when a design is technically available.