2008 Ford Focus.What car?
On mine it's 20MPH but A sneaky trick if you so desire to have CC below 30MPH (quite possible on long stretches of 30mph roads in some towns) is to activate at 20MPH or above then use the steering wheel control to drop speed, it drops to 15~MPH and you can use the return to cruise speed button thereafter.
I haven't really tried it but the manual states that it will not work under 50kmh, I will try it when I go home in a few hours.