Will almost certainly be due to winds. Its 150+mph at the moment if you go further north. Average for the entire journey I would expect to be about 50kts headwind going further north. Going on a more southerly route you could expect this to drop to about 20. On a 14 hour flight this would equate to the plane having to fly through an extra 420nm of air to get to london. So about an extra hour.
The planners will look at this and also the cost of overflying, China, Russia etc which are more expensive than flying to the south. If it works out cheaper to go to the south they will. Sorry to tell you this but it will have nothing to do with getting you there on time, it will all be about which way is cheaper to go.