Flanker, if you have a way of predicting holding at all the UK airports you listed 15 hours ahead then I'm sure lots of people would love to hear from you and may I please have next weeks lottery numbers! I suspect Antiguas post regarding no go-around at all may not be quite as he intended as in this situation I think the ASR from the P2 and the mandatory report from the engineers would reach him much quicker than an MOR.
Spitoon - my book estimates extra fuel burn at 4%/hr, thus over a 12hr flight you'll burn 48%of the extra fuel to carry it. Assuming you'd like about 4 tonnes spare at the start of the ILS, thats 8 tonnes extra you'd need to load in Singapore, which is about 80 passengers. Longhaulers make extensive use of reclearance, whereby they file to a nearer destination then refile onwards towards the intended destination as the fuel situation permits. However nobody can think of everything, the best laid plans etc etc and sometimes you may find yourself victim of excessive, unpredicted holding or you may turn out of the hold to be given a 40 mile final. Even with that extra 4 tonnes it may still not be enough. Sooner or later somebody is going to run short no matter how much extra fuel we carry.