How quickly OPVs could follow the First Cod War

If Britain had started an OPV project after the First Cod War, when could it have entered service?

Probably on a timetable comparable to the later Island and Castle classes, because an OPV would not require a prolonged development program.

An offshore patrol vessel would offer a comparatively straightforward response to an established patrol requirement. The later Island and Castle classes give practical guides to the kind of ship that might emerge. Starting after the First Cod War would therefore shift the timetable more than it would demand a radically different development process. Britain historically kept assigning the work to frigates until their operating cost made dedicated patrol ships politically easier to fund. The technically simple ship still depends on a budget decision to order it.