Thinalbert is slightly wrong about contingency fuel. It is 5% of burn but additionally is 15 minutes fuel holding at 1500' at the planned landing weight.
Sooner or later someone is going to get caught out into LHR. It is routine to commit to landing there from the hold, and I can think of many occasions over the years where the airport has shut completely at short notice. (eg mortars, accidents). I always carry extra there, and have on a few occasions been very glad of it. Much more fuel is wasted in inefficient flying (badly planned or high speed descents) than is burned by carrying a little extra, in short haul at any rate.
IMHO the management dogma of min. fuel carriage is one of the greatest threats to flight safety at present.