Single entry visa or 72 hours free transit one?

Hi, I am travelling with British Passport to visit Shanghai from London and my itineraries are as follows. Could you please let me know if I need Single entry Tourist L or if I can use 72 hours free transit?

1) London to Shanghai - stop over at Seoul for 4 hours with Korean Airline (not leaving the airport)
2) (within 72 hours) Shanghai to Seoul with Korean Airline - Direct flight
3) (after a week)Seoul to London with Korean Airline - Direct flight

I think I read that destination and place of departure can not be the same. I am not sure if transiting place is assumed as place of departure...

Thanks,