If you are late or have missed an exam due to transport issues or for example, you did not know the location or time of the exam, then you cannot apply for mitigating circumstances.

It is your responsibility to make sure that you arrive on time, know where you have to be, and what time your exam starts.
If you are late for an exam, the exam boards will decide if you can resit the exam or not.
FAQ 27 and 713 tells you what can and cannot be accepted as mitigating circumstances and what evidence is needed.
FAQ 244 explains when the exam boards sit.