We have done the October transatlantic on Britannia a couple of times. We live 45 minutes from Manchester airport, so it works out well for us to drive to the airport and leave our car there so it’s waiting for us when we fly back into Manchester two weeks later. One thing to be aware of is that although the coach is included, you do need to phone P&O to book your seats on it. They will then tell you which coach you are on and what your departure time is. The coach leaves from the coach terminal at the airport.
On all other cruises we drive to Southampton rather than using coaches, but for the transatlantic the coach really is the most straightforward. As Megabear said, you would probably want to come down the night before from Scotland and maybe stay in one of the airport hotels.
We are on the Arvia transatlantic this October, I phoned P&O last week to book our seats on the coach down. Our departure time is 8.30am. Hope you enjoy your cruise!