Not sure what to tell you, since I already wrote that that region has uninspiring scenery. I would urge you to stick to minor roads as much as possible. France has an excellent network of them. France also has some great paper maps from I.G.N. & Michelin. It's no longer sufficient to simply avoid "N" (national) roads, because many formerly "N" roads have been recharacterized as "D" (departmental) roads. By reading a good map, you can find the quieter "D" roads. Michelin still has their maps online, so you can do some route planning now. Their website isn't as easy to use as google maps, but the maps they display are much better than google's.
I think that google's time estimates make the assumption that you never stop for any reason and you're not carrying panniers & other gear on your bike.
If you keep your train & ferry reservations to arrive in Calais, when you're headed back to the UK, a nicer route can be found by more or less following the Seine valley toward Rouen, then head toward Dieppe. I believe there's still a ferry connecting Dieppe with Newhaven. You might want to consider taking a short train ride (to Vernon, for example) to get out of Paris.