Volver asks why the Queen's coffin was driven from Balmoral to Edinburgh, taking 6 hours by road instead of 45 minutes by air.
For one thing, the people past whom she was driven considered themselves her neighbours. Her ceremonial duties were kept to a minimum in Scotland and when she was at Balmoral she was "off duty" Many of the neighbours (and tourists too) had memories of meeting her in the hills complete with headscarf and dogs, like any other Scottish countrydweller. Once she was on the plane at Turnhouse, she had left Scotland for good. As she arrived at Northolt, someone said "She's come home" as though she had died in Timbuctoo. The funeral ceremonies and "drive-past" were to be in London and Windsor, the interral at Windsor, (nothing of note was planned for Sandringham) Had she been whipped off to Northolt at the earliest opportunity, there would have been no chance for her Scotttish neighbours to pay their respects.
Why the slow drive? Because that is the respectful speed that is customary for an occupied hearse to travel. Why the tractor guard of honour? Why not? An imaginative move by local farmers, and good for them. I don't suppose it was PR, pre-organised by the Palace specifically as a marketing ploy, it had all the trademarks of a "great wheeze" thought up by some farmer who planned to stand his kids on his tractor to get a better view and expanded that idea to include his friends.
Secondly, the logistics of flying the coffin from Edinburgh may have been better than from Aberdeen. It was a big plane to accommodate. Plus, the hearse would have had to drive from Balmoral to Aberdeen anyway, and once they were on their way, they might as well drive down to Edinburgh.
Perhaps, also, it gave the family time to get home to London themselves in time to meet the coffin on its arrival? That would take a while, even for them.