Small warships as command and presence assets

Would a British coast guard taking Caribbean and patrol duties be a useful substitute for Royal Navy small ships?

Not necessarily: it could reduce junior-command experience, duplicate costs, and still leave situations that require a warship.

Small vessels give junior officers meaningful command opportunities at manageable risk and cost. Damage to a River-class patrol ship is cheaper to absorb than damage to a Type 45 destroyer. A separate coast guard would still leave places such as the Falklands needing a grey-hulled naval presence, which could duplicate spending rather than replace it. Routine patrol ships develop personnel while meeting operational commitments.