Deffo use Autoroutes. They're so much better than motorways over here.
You can average 70+mph, whereas on N roads you'll get up to 55 mph if you're lucky, and slower for every village, bend, lorry etc. And navigation is easier.
Personally, I'd use P&O Dover - Calais, rather than the tunnel.
You're going to be in the car long enough as it is, so why stay in it for the crossing?
I just started using the tunnel this year, admittedly out of season, and find it so much nicer and quicker than the ferry.
Having pre-booked I just drive up to the check-in and insert the old credit card. The display confirms my details and if I am early tells me I can take an earlier crossing, no extra charge so far. The usual border control stuff then an easy loading process. Once on you can't walk anywhere interesting but you can stretch your legs or use the on board toilet.
Mind you I can see it could be different in the holiday season and I do time my journeys to pay the cheapo fares. On my last 2 trips there fewer than a dozen vehicles on the train.
On the French side I was amused to see the tunnel route described as a peage!
Would agree about the autoroutes as being quick (and far less interesting) altho' out of peak times, not the Rouen one! Peages( toll roads) are pretty easy as you can use a UK credit card to pay (get a receipt by pressing the RHS button - The tolls on my trip to the Limousin are about 17 euros.) The rest areas on French routes are very good, much nicer than UK ones.