Hi,
I am asking this question for a friend who is flying from the US west coast to Africa with a 12 hour layover at LHR in April. His checked baggage is being transferred to his connecting flight to Africa. He is arriving early in the morning at LHR. Would he have time to pop into London and see a little bit during that 12 hour period factoring in deplaning from the US flight, going through immigration and then going through security upon return to the airport? He will have a carryon with him which I think he can store at the LHR terminal. Thanks for your help.