Why the Type 31 is not an offshore patrol vessel

Is the Type 31 merely an offshore patrol vessel rather than a useful frigate?

No. Its size, range, weapons growth margin, and task-group role distinguish it from a constabulary OPV.

An OPV is optimized for persistent constabulary work where sending a high-end combatant would waste money and readiness. The Type 31 is a much larger frigate with useful range and space for Mark 41 cells, giving it roles in showing the flag and reinforcing task groups. Strike-length cells are identified as the longest Mark 41 variant after an initial misunderstanding about their missile capacity. Physical room for a missile still does not provide the data links and combat-system integration needed to use it.