I've used both of them - P&O and Brittany quite a few times over the years. The experience is roughly akin to spending two days in the motorway services. You can do it but it becomes a bit of an expedition in its own right. You can also do it both ways as a sort of four day mini cruise but I think you'd have to into training for that one.
For where you're going it's probably just about cost effective though. You've got to get across the channel somehow and by the time you've factored in the cost of the other ferry / tunnel, fuel, hotels, motorway tolls (particularly if you do Dover / anywhere), tyre wear etc the price of the Spain ferry starts to look a bit better.