If my estimates are right, and I have not been to London since I was young, there probably is not enough time. I am assuming you are travelling to London from outside of the EU, so you will have to clear immigration and customs. Since it is so early in the morning, that might be very quick, but it seems at the very best you will make it into London at 9:00am, that gives you about 2, 2.5 hours before you have to be catching a cab back to the airport. I personally would feel too rushed. What do you think? Are my estimates off?