Why Britain divided the Type 26 and Type 31 requirements

The historical distinction is between frigates suited to independent presence missions and ships supplying the numerical strength of task groups. A more capable ship could deploy alone to demonstrate British interest without requiring a larger formation. The procurement difficulty was obtaining that Type 22-style capability at a price compatible with the Type 23's original numerical role. Type 26 and Type 31 eventually separated the requirements. Type 26 provides the more substantial capability, while Type 31 helps supply numbers. The irony is that the programme associated with numbers has the smaller production run.