Type 31 as the Royal Navy's lighter frigate

Is the Type 31 merely a frigate-sized offshore patrol vessel?

No; it is a genuine lighter frigate intended to complement the higher-end Type 26.

The Type 31 remains a frigate even though it carries fewer high-end systems than Type 26. The relationship is compared to a smaller Arethusa-class cruiser operating beside a Town-class cruiser. The analogy emphasizes relative scale within one combatant category rather than a divide between warship and patrol vessel. Type 31 therefore supplies presence and general-purpose capacity while the scarcer Type 26 concentrates on demanding missions.