When purchasing a round the world ticket, check the terms and conditions carefully. To get from Singapore to London with a round the world ticket, you may have to go the long way round, to accomplish what it says on the ticket. So for example, you might have to spend 19 hours flying from Singapore to Los Angeles, with a brief pit stop in Tokyo. Then, Los Angeles being Los Angeles, you might have to fill in a form and collect your baggage and pass customs and immigration before getting back on a plane from Los Angeles to London, taking maybe 11 hours. The advantage of this route is not always obvious to the weary traveller; perhaps going the wrong way across the international date line gives you more free meals.
Another Top Tip: long haul flying is very much like income tax. You are going to be badgered by the whole jet lag thing, so you might as well go out all night, get little or no sleep, and try to get your hangover onto the plane. That way, you can claim back the loss of sleep from drinking against the inevitable dizziness and confusion that the plane will foster in you. Its like a tax dodge, only with sleep. Only drawback is that you have to pace around airports when catching connecting flights to avoid falling asleep in a corner…