We used to do Chester to Blairgowrie in 5 and a half hours. If we missed all the busy spots with two short stops to change drivers etc.

Thelwell Viaduct is now OK most time other than rush hours.
The Blackpool turn off could be busy early if the weather is good.
Same goes for around the Lakes/Kendal/Keswick sections.
take care if you are on the bit where the M6 becomes A74 two lanes of madness!

Have fun!