Just a few thoughts to make the pool ripple a little perhaps. Have not operated commercially LGW or LHR since 1990, but corporate frequently more recently; little seems to have changed - ATC still doing a grand job as they always have, and lots of aircraft all the time, especially at morning and evening rush-hours. Big machine -2000 kg/hr, small - same in lbs. Early morning or evening rush, carry an extra 45 minutes worth; add the risk (any) of fog- a further 45 minutes. This on top of the extra necessary at busybusy times to cater for the lower levels possible en route. Planned alternate neither the other R/W nor any Londons, as the latters will all probably be liable to similar weather and very busy with everyone else likely to be diverting in these circumstances, with extended routeings resulting there to boot. Sounds ideal and is; keeps the adrenaline level low, grey hair at bay and at the end of the day even if you have burnt as much as 10 percent of the extra 3000 you put on, you have the aircraft and the folk where they should be, so they are more likely to fly with you again, and the next lot can depart on time, or nearly, so they too will be happy, which makes the extra 300 burnt a very good investment! Assuming performance allows it on departure - do it! It used to be called airmanship,it made sense and whatever any Company manual said, it was the commander's decision - and responsibility - to carry prudent additional fuel for the anticipated conditions, which included the aim of getting the folks behind you safely to their desired destination. Surely that still applies, so why even consider minimum fuel in the circumstances outlined? An extra 50 or 100 miles routeing matters not a jot or tittle with enough extra fuel planned for and carried on board. Throw away the sharp pencil immediately!