Have only just seen this thread.
You seem to have figured out pretty much which way to go, you are just looking for nice places, right?
Only thing I can add to this: We drove down to Dunkerque in April to take the Norfolk Line Ferry.
The drive down to Dunkerque was bloody long, much longer than we had expected (we only started at 10:30 a.m.). We thought we had booked a NICE hotel, but it turned out to be quite a tip. In case you should stay in Dunkerque, you might want to go to one of the hotels near the ferry port, there's a big choice, suppose every single one of them is much better than the one we stayed at.
However, our hotel was in city centre, there's also a little marina, it seemed quite nice when we arrived, but turned out to be completely dead when we went out for dinner (and it was a FRIDAY night!). Didn't see any nice restaurants, so went for food to a pub that turned out also to be the local booky. Interesting clientele...
The food was unexpectedly good, prices OK.
However, Dunkerque didn't do much for us - maybe we were at the wrong end and there's nicer areas?
We decided not to take the ferry back from Dunkerque again and rather drove to Hull (which was a much shorter drive coming all the way from Scotland with a short visit at friends' in Preston) and the P&O ferry from Rotterdam really is ace. In comparison to the other ferries we've used over the last 3 years it's the best. We invested a little more and booked a really nice cabin, which was absolutely worth it (it also came with a free mini bar - which was empty by the time we left in the morning...).
If you take your time driving back from France I can imagine there's nice places, but going on the ferry and then all the way down to Hamburg in one go - that's bound to take very long, cos sooner or later you will hit rush hour SOMEwhere...
Don't know how Lazybum got there so fast, we didn't (no, we are not afraid to drive more than 140km/h).
edit: we did get past Bremen before it got busy (had discussed the matter with Lazybum and others before) - but it didn't help.