From the UK Owners Manual page 426:
You cannot set or resume in the following situations:
• When vehicle speed is less than 30 km/h (18 mph)
• When the MAIN button is turned off (I know it is turned off as the time is displayed instead of the CC symbol.)
• At vehicle speeds of 25 km/h (16mph) or less, cruise control is cancelled automatically.
For some weird quirk, UK models do not have a button marked CRUISE to activate the CRUISE Control, instead we have to depress a button marked MAIN!
One other thing to take into account, if you have a manual, you cannot activate Cruise Control if your foot is resting on the clutch pedal - even slightly, as I discovered quite early in my ownership.