12.4 mmol what does that mean
A single one off number doesn't tell you much of anything apart from that your blood sugar levels are too high, but let's face it, if they were normal you wouldn't have been diagnosed with T2.
To gain any useful information there needs to be a little structure to your testing.
Just before a meal test and make a note of the result, then to discover how your meal affected your glucose levels, test at intervals after your meal. Depending on what the meal was, 45min to 1hr should give you an idea of how high that meal is taking you. By 90min hopefully it's on its way down, and in an ideal world at 2hr's you would be getting back somewhere close to your starting level.
If at the 2hr mark you are still 2mmol or more above your pre meal level, then there were too many carbs in that meal for you to handle and you need to consider how to reduce the carb content of that meal, or even maybe cross it off the menu altogether.
It means a lot of testing and probably sore fingers but in a few weeks you will have a pretty good idea of how you need to change your diet. Keeping a food diary along with a record of your reading will help.