Can I add my twopennyworth. Take the long ferry, it may be expensive but so is driving down thru France when you take into account peage/hotels and fuel. Crossings can be choppy but can also be a delight. The drive down to S Spain from Bilbao is brilliant, we have driven down to the Algarve several times in recent years, albeit not MX5. Also - almost toll free, at least it was several years ago and the roads are great. Plenty of stopping places, we favour Salamanca for an overnighter. Enjoy!!
As an alternative, how about the ferry into St. Malo rather than the chunnel? We go to France 3 or 4 times a year and almost always use Caen or St. Malo depending on destination/timings. As a starting point for Spain, it knocks a big chunk of mileage and toll-fees off straight away.
Brittany has no toll roads, so the first part of the journey South is free, then the N137 runs parallel to the E3 route for many miles, so you could choose whether to do one or t’other, or a mixture. We use Bain de Bretagne on the N137 as a frequent stop on our trips, as it has a good selection of eateries around the Centre Ville and a supermarket on the outskirts with cheap fuel.
As far as motorway cruising goes, the MX-5 isn’t as good as some (our Jaaag was way better!) but is fairly civilised with the roof up - we rarely go top-down on the motorway as we find it quite tiring and it murders the fuel economy! I have, however, eliminated the sound-tube-thingy that the NC has and that made quite a difference to noise inside the cabin.
Wife and I have been frequent visitors to France in our NC 2.0 BBR modded hard top since 2018. My wife does not drive and is a lover of exploring locations whereas I enjoy the route planning then the driving of them.
We have an Emovis tag but now rarely use the motorways or even the fast N roads - conversation in the car at motorway speeds is difficult, the scenery generally boring and wearing the MX5 after an hour a bit uncomfortable. We have our MX5 for fun and as one poster already pointed out you don’t get a MX5 to drive straight lines. And boy there are plenty of MX5 grin roads through the French countryside if thats what you want. Some are great for testing the traction control light!
However, over the years we’ve learnt that to enjoy the holiday:
France is not a racetrack with the beach as the finish. It has an incredible and varied geography and best enjoyed at leisure.
We stay in family hotels rather than international chains. Hosts generally are friendlier and the places can be unique (quirky?).
Contact the hoteliers in their language and see if they would accept a direct booking at the cheapest comparison site price. French like a bargain so not keen on paying agency commission.
Stay two nights at each location so you get time to have a good look around.
Plan for two and a half hours driving time (or 200miles) between hotels. Allows for a couple of coffee/sightseeing breaks. Ignore the French 2-hour lunch and look forward to a good evening meal.
Decent cafes can almost always be found in village/town squares where you can often sit outside and look like a local. If desperate, we have never found a hypermarket yet that does not have a good value cafe and toilets.
We have a Starling current account which does not charge for foreign use.
Get a Blue parking disc (ebay). Often needed for even the quieter towns for roadside parking or their only carpark. Noon to 14:00 hours normally free but parking disc would give you an hour extra before or after this.
Don’t leave an Emovis tag visible in your parked car. Its linked to your bank account so can be used in any other car.
In France it is illegal for the vehicle driver to use a mobile telephone for any purpose on the highway and that includes a smartphone contactless payment for tolls.
Get a Crit’Air disc as more and more French cities are requiring them (Caen I think was just this year)
A place name sign always indicated the start and end of the urban 50 km/h limit. Treat it now as the start of a 30 km/h limit. Since our first drive in France in 1980, main roads to ferry ports have always been the hunting grounds of enthusiastic speed checks on returning Brits. In a trip last year, we cleared one where an unmarked Police van was parked about 20 yards past the town sign and backed up to and obscuring a 30 km speed sign. The local PC was 20 yards after the 30 km sign with a hand-held radar. He got a wave and smile as we passed - it was not returned.