So many people lead such very busy and pressured lives these days, particularly in the UK. This takes me back to a memory of one student who would arrive into the car park driving with one hand with his phone in the other. With seconds to go he would finish his call, grab his flight bag and arrive at the flight desk bang on time. At the end of his lesson within seconds he was back in his car, driving out of the car park one handed, and once again his phone in the other. That was his life.
I'm certain that he wouldn't value a patronising lecture from me. The answer is as suggested add a payed for pre-briefing of say 15 minutes and a post flight brief of 45 minutes. Take-off on time and fly the full duration. There can be no justification for a loss of instructing income. Such a person will fully appreciate this because its how they no doubt earn their living.