considerable.

Brigthton to Gretna is a distance of approx 375 miles( according to my map).

Joseph Ballard ,an American who wrote of his experiences travlling in England in his journal " England in 1815,being reflections and comments on a trip through Britain in th e year of Waterloo " wrote as follows;

I took a passage on Saturday in the coach for Warrington eighteen miles distant from Liverpool.......There were six pasengers insdie,and 12 outside the coach, besides the coachman,and an abundacne of luggage.This added to the weight of the vehicle...makes it almost incredible that they should be able to go at the rate of 7 or 8 miles per hour...."

So if we just consider the mileage alone and don't consider the neeed to stop to change horses, and for refreshment etc, we have a bald figure of 46.8 hours, if this sensational speed could be maintained for the lenght of the route.

Louis Simon , another American travelling from the West Country to London by private coach in 1810-11, in his "Journal of a Tour and Residance in Great Britain" (1815) gives 6 miles an hour as the usual rate of travelling.

I suppose, a rate of around 4-5 miles per hour would be more realistic, making the journey at least 75 hours long( not including necessary stops)

The journey, I strongly suspect, would take much longer than that of course,as the post road system was much better then what existed previously,but were still subject to variation in quality,and comfort stops would have to be taken( as previously stated)