How Far?

Outside the Highgate Hotel which I am guessing was an old coaching inn and with both sons currently living in Edinburgh this mileage stone has an interesting fact.

On the other side of the steps is a stone which says London 258 miles.
The building is 17C , I wonder which roads the measurement was taken along in those days.  Google Maps offers 272 miles via M6/ M40 or 263 miles M6/M1. Interestingly the M6/ A701 route to Edinburgh is 138 miles- nearly spot on. And how did they measure it in those days anyway???