Hello and thank you for using Just Answer,
You are correct the publication does not specifically address a lay over but you must look at the specifics of your physical location. The rule is to be not just outside the US but physically in another country. The 16 hours you would be in international air space would not count because you are not in another country. A foreign country usually is any territory (including the air space and territorial waters) under the sovereignty of a government other than that of the United States. You are allowed to move about between countries and not lose a day in your counting as long as you are not in the US for 24 hours. In taking the rule at it's face value, you would need to be in the US for less than 24 hours during that time. In your original question you asked about when the clock starts. It starts when you leave then midnight would start a new day. If you have a layover in UK then that still means you are outside the US and in a foreign country. No loss of time there.
To simplify, if you are over international waters for the 16 hours then that is not counted against you because you are not leaving the US nor in the US for more than 24 hours. You would not lose a day in your counting. I can point you to court cases and rely on my experience to further the interpretation.
As I pointed out above, if you are in transit between two points outside the United States and are physically present in the United States for less than 24 hours, you are not treated as present in the United States during the transit. So, if the trip, including the stop-over in the U.S., takes less than 24 hours, you do not lose any physical presence test days.
My interpretation is based on experience in the Form 2555 with past clients that traveled.