I always use the included coach from the far North West. As others have said ths ship will wait (or if not it is up to P&O to sort things out). The drivers are in constant contact with base and each other, and often go a different route according to the info they receive. Never had any problems. There will be one or two comfort stops, and short pick up stops along the way. Yes it's an early start, but who sleeps the night before a holiday anyway? Usually arrive early afternoon (between 1pm and 2pm). You give the luggage to the driver and you next see it outside your cabin.
You are more likely to suffer traffic delays driving yourself without the info that the coach drivers receive, and no staying in a hotel the night before with the commensurate luggage hauling, not to mention two sleepless nights prior to your holiday.
Eavesway is classed as independent travel and P&O will not take any responsibility for problems (such as when luggage was off loaded at the wrong ship, no compensation etc). Also, their pick up points are much more sparse, (no farther north than Preston on the west side of the country, none in Scotland).