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Yes - likely to be milder by the coast so should be more pleasant riding than inland. Obviously, book last minute after checking the weather forecast for nasties, campsites won't be open so hotel or bivvy if you are hardcore, and an early start from the Dieppe ferry would involve quite a long, dark morning as the sun won't be up until about 9am.
Ferry schedule a bit restricted in winter so fewer options, but still plenty.
Having done exactly that in September, I would seriously question the experience and judgement of whoever told you that!
It's a beautiful coastline, stunning beaches, great restaurants lovely campsites, and virtually all of it is either on cycle paths or quiet lanes.
I did it over three days - two of which were really half days. First day was train to dover and ferry. Good chat with three other cycle tourists on the ferry. Then 25km afternoon / evening ride to Wissant. Lovely coastal scenery.
Long day was 130km to Le Crotoy on the Somme Bay. Really beautiful beaches. A slightly dull couple of hours in the Netherlands sense section, where there was a long section of rural cycle path with not much happening, but lovely views most of the day. Campsite right in the town at le Crotoy.
Last day was flat across the coastal marshes then hilly along the cliff tops to dieppe. Beautiful little seaside resorts for coffee / lunch stops. Then dinner / final beers in dieppe before the ferry.
Id really recommend it for a relaxed trip. Look out for the wind, though. It's normally going to be a tailwind from dieppe to calais so default would be to go that way. I went from calais as there was a N/E wind that week.
Otherwise I've done loads of trips from dieppe and really recommend it as a base for a circular tour. I've generally done long rides but have in mind some circular Normandy trips, combining going west along the coast and back inland (or reverse if wind). Hard to go wrong route wise. The only bit I don't like is the Voie verte. I think you can have too much of them, like canals.
I'm sure further west is lovely too. Logistics of Dover and Newhaven are very easy from London though, and the ferries are cheap / can book late. But I'll work my way down there over the next few years!
https://ridewithgps.com/routes/44529008
I'd definitely do it again, maybe the other direction, different stops, different side detours, etc.
I liked it so much I took my family back to the Somme bay for a few days at October half term, and did a bit more riding around there.