To cut to the chase, if you really want to go to Spain, you only have 4 weeks and you value every day, I would take the ferry. Let’s face it Wully you can afford it! They have a monopoly, but they get the trade. We had to pay extra end September as we left booking late as we were unsure of dates. We love France, but the thought of driving though it, just to get to Spain, after driving though the UK, is mind numbing. You’ll arrive in Spain raring to go and with a lot of options where to go. There is a wealth of POIs along the coast in lovely places, many of them free and with services, and lots of interest to be had if you choose to go inland. You can cover a fair bit of ground with the best part of 4 weeks, even taking it easy. Oh and get a cabin. We once did the seat thing between Rosyth and Zeebrugge, never again!