Only you know what is best for your child, but my advice remains to get two seats, in Y if you must. Frankly, a trip of this length (I'm presuming LON-BKK/KUL or similar based on your OP) can be hell with a kid of that age even with two C seats. Sometimes it goes swimmingly, other times, no so much. If the latter happens, and you are alone in only one seat, god help you. I would never attempt it with a lap child of that size.

I guess the good news is since you're doing this several times, you can see how the first one goes and adapt.

Also, (not that you'll likely have much choice) if your child is a good sleeper, I strongly recommend booking overnight flights: it significantly reduces the amount of time you have to try and keep the kid entertained/restrained, and increases the chances you'll have a moment's peace for yourself.