All large aircraft, especially the big one's, have to trade fuel against payload. Take a B747-400. Max Take Off Weight from Heathrow is 396,890 Kg (if memory serves). Its maximum Zero Fuel Weight (ie weight before refuel) is 251,740 Kg. That leaves 145,150 Kg for fuel.
The total fuel capacity of a 747-400 can be from 163,700 Kg to 173,700 Kg depending on version. So, to get the max fuel on board you have to reduce the available payload and vice versa.
It's minimum landing weight will depend on the availability of a diversion airport at the destination but will, generally, be about 12,000 Kg of fuel. Therefore, a B747-400 will have a max fuel available to burn of between around 133 Tonnes and 160 Tonnes depending on payload.
At a True Air Speed of 500 knots (a realistic sort of figure, though due to winds you can get ground speeds up in the 600 kt mark going Eastbound and down to 400 kts going Westbound) and an average burn of 13 Tonnes per hour, you get a range of 7,500 nautical miles (about 8,600 statute miles or 13,900 km) London Singapore is getting on for a 7000 nautical mile flight, with wind that's an equivalent of about 6,500 miles London Singapore and 7,500 Singapore London.
Ergo, if you follow my ramblings, Singapore London is just about at the limit of a 747-400's useful range and, in fact, in BA is the most weight limited route - by range (though places like Mexico City pose their own problems due to the massively reduced maximum take off weight at altitude and high temperature).