Best route surely has to be A1 then go up the A697 - much quicker than the A1 through the borders
Personally I prefer A1 then A696+A68 but I have not been along the A697 before.
Too late now but one solution could have been fly to Newcastle then take a train (or even rent a car there).
Flying to Glasgow could also have been cheaper (or even Aberdeen).
For people who collect Avios, BA effectively caps the price of flying short flights as Avios tickets are frequently available last-minute for domestic flights, unless it is something like the first day of school holidays or bank holidays.
If train prices are in the £300s it might be cheaper to open a new BA account and purchase a small amount of Avios at a rate that one should normally never pay.
While the USA is a large country, the vast majority of air travel is sub-300 miles. Just look at the number of flights between city pairs like Boston-New York (over 80 direct flights each way every day) for a 220 mile drive. If the airline/car industry hadn't done such an effective job of killing passenger rail travel then easily 60 to 70% of current domestic US flying wouldn't exist.
Some of this will be connecting traffic, and I think that 60-70% figure is dubious, I would estimate that even if the US was covered with high-speed rail lines, domestic flights would only be 25-50% lower than today.