Great North RoadThe Great North Road
This ran from London to Edinburgh and was the principal route north. Modern highways still follow some of the route, which in the early days of modern roads became the A1.
There's a still existing junction called Scotch Corner. That's where an alternate route to Scotland goes off to the west because Scotland dips down there and is closer. The border point is Gretna Green, which is why it became linked to clandestine marriages. It has no other significance. Any point in Scotland would do.
There were variations at points, and I'm slowly pulling together the details.
The London Starting point for measurement of distances was Hick's Hall.
This part is in the north and relevant to An Unlikely Countess. It's from a 19th century source, but the basic route wouldn't have changed:
206 Boroughbridge (In 1745 a turnpike [toll road] was built between Boroughbridge and Darlington)
216 Sand Hutton
220 South Ottrington
229 Lovesom Hill (?)
231 Little Sneaton