Again without knowing your company's interpretation of CAP371, you aren't actually losing the 30 minutes duty time.
What happens is that your maximum FDP is based upon the flight crew's report time, not yours. Your 30 mins still count towards your total duty hours, but those hours are based on the time the flight crew report for duty....
ERM... I think! Take a look at your General Manual Part A, or your flight time limitations handbook (if you have one).