Assume, as said by others, that it is a 9hrs plus drive…but on rare occasions it may take just under 9hrs.
You will always need to take a driving break in each direction.
Any slight hitch occurring after the time you would have had a break, then you are over the 4-30 and you are in the wrong.
Taking that driving break will inevitably put a couple of minutes on the drive as you slow, pull off, park, accelerate and manoeuvre onto the road again. (see above: assume always 9+hrs)
Really, it`s not worth chasing the under 9 time.
If that sounds a job you want, then fine.
If you think you will need more shifts and need the 3rd shift, then look elsewhere.
As already said youll likely need a 45 on the way up, and if you can get a 15 in Glasgow, Youll be OK with a 30 on the way back.
Id try to get past the 2hr30 or 3hr mark on both drives, but dont see any advantage to going much further.
Assuming you will be doing over 4hrs30 each way, then many prefer to do a 15 break after a while, followed by 30, later on. Personal preference. See how you feel on any particular day.