a 68 is anything but stress free,how long is it since you drove on a68.there are that many speed cameras now its a nightmare at times??????Being a regular traveller from London to the Highlands, the route I would use if time permits, is.
M8 to Edinburgh, then the bypass, A68 south to the A1M (just North of Scotch corner. Then A1 south, A14 at Huntingdon then M11 and M25 (clockwise). The A68 is a lovely drive takes abound 1hour longer but is stress free driving.
Alternatively if time is a major factor but you want to minimise the M25 take the M74/ M6 south to the Junction of the M1 then take A14 to huntingdon and then as above.