Having tried various routes with three big Weimaraners, the least stressful for ALL of us is a sedate drive down through France. My friend from Dublin uses the Fishguard ferry, then drives down to Poole for the 4 1/2 hr Cherbourg crossing ( which is the one I use to get back to Cornwall). Really, taking your time driving through France with many dog- friendly hotels and plenty of places to stop and exercise en route is actually a joy and the reason we choose to travel and not huddle behind the front door in England
Good luck
Ruby