Why are more Royal Navy patrol vessels unavailable for Channel duties?

Why are more Royal Navy offshore patrol vessels not available for border patrol in the Channel?

They are already stretched by other commitments, while border cutters were funded to perform this work. Giving the task to the Navy requires additional ships and money.

The Navy's offshore patrol vessels are already stretched by other commitments. Border patrol is a separately funded responsibility for which cutters were provided. Moving that work to the Navy raises the question of where additional ships and money would come from. Assigning it without extra resources would duplicate another service's task while increasing pressure on the fleet.