I am starting to think about possible tours for next year and one of the options I am considering is to fly out to Biarritz and cycle home mainly along EuroVelo 1 (La Velodyssee). Does anyone have any experience of this route? What are the surfaces like? Is the southern end interesting or just flat and foresty?
If you can stand the early climbing there is a ferry service from Portsmouth to Bilbao. Hard work from there to the border but then it's OK.
We have not ridden the actual route in extreme south west France but there are lots of cycle route crossing Les Landes and they are indeed flat passages through pine forests growing on sandy soil. The middle picture of the Wikipedia entry for EV1 looks very much like the crossing of this area we made going south in 2017, tracking somewhat further east. It was possible to get better speed than on most cycle paths. We crossed Les Landes in one big day mostly on cycle route. The sound of the pine needles under the tyres is distinctive.
We've only ridden tandem across the Vendée on D roads but the very coastal route looks interesting and probably slow especially through the dense coastal resort areas. A lot depends on how many tandem unfriendly barriers there are. The barrier details seem to be under the control of the département so can be more or less difficult on the same signed route.
We first rode tandem along the Nantes Brest Canal section from Carhaix-Plouguer heading south east in 2001, new to tandem touring we struggled a bit with some path wash outs from a wet spring and were not well set up for the heat. This highlighted one of the common problems using this kind of route - the need to divert off route to find refreshment or in this case just water. We rode much more of the Canal path in 2017 coming from the south. We had time in hand before our ferry back to the UK which is why we joined Velo Verte routing at Châteaubriant. We found the southern section of the canal much more like a UK canal route with occupied lock keepers houses decked out with flower displays and more cycle usage. More experience, less luggage, more water bottles and more appropriate mapping made it a lot more fun.
Out of curiosity we followed the Velo Verte Carhaix to Morlaix right at the end of the 2017 tour. We were glad it is mainly downhill going that way and we were in no rush as there were some very energy-robbing soft surfaces.
Although these pre-cooked routes seem to offer total freedom from navigation, we have been a lot better able to cope with whatever happens with full mapping available. We use Viewranger on a smart phone with paid for (premium) maps. We have a "for life" IGN 1:100,000 for the whole of France but the current subscription deal doesn't look bad in the context of a major trip like this.
We cycled southwards from La Rochelle to Rochefort this year and weren't very impressed (being on a recumbent tandem trike we had to leave the route a lot because of boulder barriers) - and quite a lot of tatty coastal strip. Have you thought of the Manche to Med route (CTC holidays do it)? We have used the Bikebus to facilitate linear and circular rides in the southern half of France - brilliant. We also find hotels where we can leave the car for 2 weeks. One of our first circular routes from Orange was such a success that we repeated it some years later - happy to share the route.
When you land at Biarritz airport, head for Casino in the centre of town. There you can pick up the coastal cycle path which you can follow along the coast to Bayonne, across the Nive river and then across the Adour river. I am pretty sure it is reasonably well signposted. Biarritz is well worth a visit depending on how many tourists are around. The cycle path is not made for tandems and can be rather narrow in parts.