Many cars do not allow cruise control to be activated below 25mph or 30mph, as in most instances it is unlikely you would need to drive at a low fixed speed such as this for an extended period of time, unlike a motorway where you could expect to maintain a fixed speed for some time and distance.

Alternatively if you car is fitted with a speed limiter or warning function you could set this to chime in at 21mph, giving you a safety net to avoid exceeding the speed limit.