Why early postwar frigates kept plain frigate labels

Why were Royal Navy Types 41, 61, 81, 12, and 14 not generally called guided-missile frigates?

The category was not established when the designs emerged, and the ships later carried too little missile capability to fit the developing guided-missile-frigate model.

The guided-missile-frigate concept was not mature when these postwar British escorts entered service. By the time the label became meaningful, their missile fit did not resemble the ships receiving that designation, so the Royal Navy simply retained the older frigate classification.