From South Ayrshire we always user the Dover Ferry, I've looked at Portsmouth and for us it didn't work out.
According google maps its only an extra 1 hour drive to Dover rather than Portsmouth, in France from Calais its an additional 2 hours driving to somewhere like Tours, so 3 hours extra driving in total. Dover crossing is 1.5 hours, Portsmouth 4.5, 3 hours extra. So additional driving or longer crossing, time wise they cancel each other out, so that then brings in the ferry cost and the number of crossings.
For us Dover wins every time.