Sounds like the reason your Mini Cooper failed the smog check was due to one or more of its emission monitors not being ready for the smog check. In most cases, all the emission monitors should run during normal vehicle operation over the course of 200 to 300 miles. However, due to different driving habits, the criteria required to run a monitor may not always be met.
Following the Mini drive cycle below should allow the emission monitors to complete. The drive cycle must be followed accurately. Take extra caution when performing the drive cycle on public roadways. Perform the drive cycle at your own risk. If you feel you might not be able to perform the drive cycle safely, or obey all traffic laws, seek the assistance of a smog check repair station which will be able to perform the drive cycle on a dynamometer, utilizing specialized OBD II drive cycle readiness tools/scanner/computer.
Your Mini Cooper (including Clubman model) must have all of its emission monitors READY or COMPLETE in order to pass the smog test, except for the EVAP monitor (as of the time of this post). Emission monitors, also known as Readiness Flags, are internal self tests which your vehicle's ECU (engine control unit) conducts to ensure its emission systems are working properly. Typically driving 200-300 miles in both city and highway traffic conditions is sufficient to complete the emission monitors. However at times, a vehicle owner's driving patterns do not match those required for the emission monitors to run; and performing the drive cycle's specific driving pattern is required.
Since you mention having recently received service from a Mini dealer, we are curious to know if at any time during the service, your Mini Cooper's battery needed to be disconnected, or perhaps the battery was replaced. Replacing the battery or having disconnected the battery would have caused the engine control module (ECM) to reset and subsequently clear all the emission monitors (and trouble codes; if any existed)... requiring a drive cycle to be completed prior to a smog inspection.
As mentioned above, typically driving 200-300 miles through various traffic conditions and over the course of a few days is enough to get the drive cycle to initiate; and readiness monitors to become READY or COMPLETE. At times however simply driving an extend length of time does not make the monitors complete. We're going to recommend following the drive cycle indicated below. Please ensure at all times during the drive cycle that the fuel tank is between 15% to 75% full. Certain monitors will not run if the tank is near empty or full.
How to Perform a Mini Cooper Drive Cycle?
1. Start engine and idle while cold for 2 minutes and 10 seconds.
2. Accelerate to 20-30 miles per hour and maintain steady speed for 3 minutes and 15 seconds.
3. Accelerate to 40-60 miles per hour and hold steady speed for 15 minutes.
4. Decelerate and come to a complete stop. Idle engine in gear (DRIVE) for 5 minutes.
Note: The drive cycle above will terminate if engine RPM exceeds 3000 at anytime, if road speed exceeds 60 mph, or if there are large fluctuations in throttle angle.
After performing the drive cycle at least twice, check for trouble codes and ensure all emission monitors are complete. You may conduct this test at home with an OBD-II scan tool availalble from a local auto parts store. If all is well, proceed with the smog check.