You forgot the best option: get your child a seat in club, bring a cheap lightweight car seat (in the US we have cosco scenera, they're like 3.5kg), then 14 E/F will be all yours.
I would not pick the middle E/F if traveling alone (granted I have not been unlucky enough to do so). I would pick a back bulkhead seat like 15A or K or 59 A/E/K to trap your son in. Whichever seat has the most storage obvi given the need for nappies, wipes, toys, books, etc. You know your kid better than I do but you cannot rely on IFE alone to keep them entertained.
In terms of sleep, given HKG-LHR, I hope you have a very good sleeper (or he sleeps in bassinet) because sleeping with a lap toddler, short of the most extravagant SQ/EK/EY F setups, is still difficult in J. The best position that I have found is the seat in Z position with toddler lying on you. A baby carrier like an ergo or stokke would work best, but is technically not allowed during takeoff/landing on many airlines. Second best is lay the seat flat and sidesleep next to them, if you are thin enough - I can barely do this.
Furthermore, as I just flew with my little one - when the seats had to go up for landing, our son woke up and terrorized the entire J (and probably F) pax and cabin crew for the last 15 minutes of the flight due to being woken up. Well worth having a car seat where baby stays sleeping and you can literally just carry him in the car seat out of the plane.