The only overnight ferry we could do would be Rotterdam/Bruges to Hull and it's much much more expensive, even without a cabin. Dunkirk - Dover is the longer one, 2 hours. It doesn't really matter if we do Dunkirk or Calais, we'd just pick the cheapest/one with availability.
Luggage is difficult because getting to the airport here we won't all fit into our small car, once DS2's pushchair is in there it's full, only space for suitcases on laps. so would have to do train (changing twice, wearing bulky winter clothing), and then we'd need to hire a car or work out how to get picked up/coach on the other side, which would leave us less mobile when we get there, and costs start to mount up. Also flight costs are getting higher now and we'd have to go to a less convenient airport, which adds time. Lastly I'm a bit iffy about taking the car seat on the flight - I don't mind so much when it's a plastic booster but I'm worried about damage. It's not a flight appproved one so can't go in the cabin and weighs a ton anyway. We would need buggy + car seat + travel cot, he's too big for the carrycot. Could do that by spreading allowance between both DC but is a pain to transport stuff.
Just looked up luggage courier and it takes days so not really practical. We'd be without for half the trip. But I hadn't heard of that before so interesting suggestion, thanks.
I realise that a 10+ hour drive sounds like a horror to most Brits but as RedDwarves said, when you live somewhere with further distances it does seem a bit more normal. Indeed, most (all, I think) of our UK friends here tend to drive back once they have 2+ DC. They only fly when it's one person. We are good for entertainment etc as have various devices plus music plus car window games. Once we get to the UK we get DS1 to calculate all the speed limit and mile signs into km, keeps him busy.
In addition driving means we can take as much as we can fit in the car, rather than what we can take in hand luggage, and fit presents, food, drink, etc we are missing from home for the way back. The time it takes isn't that much more. Once you've worked out getting to the airport, parking, checking in hold luggage, passport control, picking up hire car and then driving to the place that's also around 10 hours, and at least when driving we have the flexibility to stop whenever we want, don't have to cart luggage around in the cold or rush around stressing about missing things or stand in security queues (or whatever) while the baby needs a feed/change at an inconvenient time. We are getting a good deal on the car hire through DH's work so it is significantly cheaper, which helps. In the future we'll get our own car to do it with.
I'm well versed in the reasons for the car seat time limit so I'm good on that one. Thanks for info :) I probably would be happy stretching to 2.5h or the occasional 3h if we're stuck in traffic etc. I know it's long uninterrupted sessions which are the issue. I know he gets fed up being in and out of it all day though. Hence 2 days feels a bit worse than one long one.
I think the tunnel is looking more doable, though if it takes around 2 hours it's not much quicker than ferry which is crossing time + 1h15, so 2h45 for Calais or 3h15 for Dunkirk. But flexibility is helpful. If we think about contrasting it with the cost of a hotel room, and we can always have a nice long stop on one side or the other to simulate a break. I thought you had to stay in the car for the crossing, though? I know on the ferry you get out and walk around.
Sashh good point about planning breaks to coincide with busy traffic. I remember we came to this conclusion last time while stuck in Brussels commuter jam.
Indeed it would be easier just to make a bed of the back seats - but obv won't do this safety wise. Good idea to switch seats with DS1 to keep an eye on the baby, and the changing mat for stops.
It's about 1000km altogether, a bit less than 700 miles. Agree it's our experience the traffic and motorways in the UK take longer than other European countries. As the speed limits are higher and they tend to be better maintained, except for parts of Belgium.
Thanls all :) Lots to think about.