I should also point out that although the captain has the final say on the amount of fuel that is carried the first officer should be included in the decision making aswell.
Yep, I don't know many captains who decide on a fuel figure without consulting with the FO. Indeed, quite a few delegate the choice of fuel load to the FO if it's his sector. Regarding the decision making process, although the planned fuel burns in our plogs are pretty accurate, and if anything a little conservative (it's rare to for actual burn to exceed planned, and we can often save a considerable amount through short cuts or slowing down) as
Chesty points out, they are a legal minimum, and don't include delays for weather etc.
Even on a gin clear day, somewhere like Gatwick can go from 'no delay' to chaos in the blink of an eye – all it needs is someone to burst a tyre or eat a seagull on the runway, or declare an emergency. When that happens, the (few) pilots who insist on flying round on minimum fuel don't look quite so clever.