I couldn get in because of the rail strike / roadworks. Can I take the exam at another time?
This depends on whether the problems were predictable before the day of your exam. If the likely effects were known beforehand, you should have made arrangements to get to the exam on time, for instance by leaving earlier. However, note that if you arrive within 30 minutes of the start of the examination, you will be allowed in (but will not be given any extra time).