More Type 31s still need a defined mission

What would the Royal Navy gain from building as many as twenty-four Type 31 frigates?

That depends on their assigned role; presence ships, OPV replacements, mine-warfare hosts, and missile trucks impose very different costs and capabilities.

The first question is what twenty-four Type 31s would do. Presence and flak-frigate work are useful, but a larger order needs an operational case for that many ships in those roles. Replacing OPVs or minehunters is possible, although Type 31 would cost more to operate and would be compared with patrol vessels carrying roughly 34 to 50 crew. Alternatively, a Type 31 missile truck could let Type 45 sensors and command systems direct its weapons remotely. That arrangement would require funded research, integration, and confidence that the network worked under operational conditions. Adding cells, guns, or other systems changes the design, price, and crew requirement, and the fleet must identify what other spending would fund the increase.