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I've got a train over to NL to ride RaTN. You could also get a ferry from Harwich to Hague I think and ride Dutch bike paths.
Personally, I prefer Spain. The roads aren't traffic free but when you're out of towns the roads are emptier and the drivers better/nicer than pretty much anywhere else I've been.
That's for road stuff but there's going to be plenty of gravel routes that are primarily gravel. I know there's a Eurovelo route that shares trails with the Pirinexus a signed gravel route https://www.viesverdes.cat/en/pirinexus/
If you can get to southern Spain there's Euro Divide which uses some known walking and riding trails, stuff like Transandalus, Al-Andalus and https://bikepacking.com/routes/bikepacking-transnevada-southern-spain/
https://www.youtube.com/watch?v=dZ82SHiAQFw
Also routes up here
https://www.youtube.com/watch?v=86fYuTEfe8Y
We've ridden down here too and it's very low in traffic being one of the most sparsely populated areas of Europe: https://montanasvacias.com/mas/ though I'm not sure how to get there with a ferry trip, trains maybe.
Ah, I've just noticed the Francaise thing. What about chunking bits of French Divide route off?
Or ask @frank9755 about his French trips
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flat(ish) and days of 50 miles or less
The Netherlands.
15 years ago I did a lovely and very easy loop starting from Hook of Holland. We stopped overnight in Leiden, Haarlem, Weesp (a small town just east of Amsterdam), Zeist (just west of Utrecht) and Gouda.
Very flat, virtually no traffic, pleasant countryside, loads of canals, pretty towns to stop off in, hotels not too expensive.
There is good riding from most of the French channel ports too, but things are a bit more spread out so not as many options on where to stay. A couple of months after the NEtherlands trip I did a similar easy tour of the very tip of Finesterre, starting and ending from Roscoff. I'd recommend that too.
Round Dieppe is nice but the coast is a bit more hilly and there's nothing that interesting close inland where it's flatter (although I could be doing it a disservice), Calais a bit flat and not that pretty. Don't know the other port hinterlands too well. Dunkirk can work as the end of a loop starting in Hook of Holland and going south through Belgium (I've done that one as well, with a detour to Westvleteren monastry!).
Northern Ireland is worth a thought. Overnight ferry to Belfast and you are quickly onto pretty quiet roads, and you can do a loop round Ulster, or a bit of it, depending on time.
If I had never been to the Netherlands I would recommend you go to France, but as I have, I think the Netherlands would be close to the perfect option for what you describe.
I've not ridden my bike for years, a long succession of being punted off by cars will make you twice shy I suppose, but find myself yearning to get away for a low stress bike tour. I'm hideously unfit now so want it flat(ish) and days of 50 miles or less.
Has anyone any experience or suggestions for routes of any length that spend little time around traffic, ideally accessible from one of the Channel ports? Canals or EuroVelo, camping or B&Bs, I'm keen to hear of your good experiences.
I'll kick off; some time ago I spent a long weekend going down the Canal d'Ille et Rance from St Malo and remember it being very pleasant with nice gravel tow paths, stereotypical Breton towns and villages all selling Kouign-amman and punchy cidre, and lots of campsites dotted along the canal to just roll up to and pitch for the night. I'd happily do it again, but variety is the spice of life and all that.