Thank you all for very useful input! Some questions/topics that several of you have touched upon:
Travelling and sleeping arrangements:
We are quite fond of public transport, but for long trips like this we like having the freedom of going wherever we want, and the comfort of a home base with all of our stuff. The van is fully electric, som mpg doesn't really compute. I did try to calculate it once, it it turned out around 120 or so. But the cost per mile depends on what type of chargers we find. Some places provide free charging, other places it is more costly than petrol. Overall, the cost of the ferries is usually our largest transport related cost.
When we travelled around Iceland, we slept all nights in the van. That is no longer a comfortable option. I'm guessing we could do at least 10 % of the nights in the car, but the tent suggestion is a very good idea that could increase this. We also have a very nice hammock (Hennesey) that could function as a tent if we find a couple of nice trees.
Thank you katekat for the tip about the Landmark trust. There seems to be some affordable options too, not just luxuary ones. I’ve also tried Youth Hostels before, but I’m not sure how well suited these are for kids? I do have some hotel points from job travels, so we could also use those. I did stay at a college dorm once, that was both affordable and comfortable, and the location was excellent.
Focus:
It is getting increasingly clear that the problem will be to limit ourselves. There is just so much to see and do, and even with almost two months, we can’t do it all. The girls are easy to please, and it sounds like there will be plenty of opportunity for DD9 to both visit art galleries and draw stunning views, and for DD11 to see some of the Harry Potter related stuff. Thank you very much for the tip about the train from York getting in at King’s Cross, Playing with Fire! That would be an excellent start to a day in London. We could easily jump on the tube from there and get to South Kensington or Kew garden. Durham Cathedral and Glenfinnan viaduct also sounds like a good plans.
-Going by the Nordic/Viking angle is a very good idea. Lindisfarne is a central part of our history here in Norway, and it would do the girls good to be able to connect that with the other parts of the old history of the British Isles, like the picts, kelts and romans. The optimal route then would be to visit both Shetland and Isle of Man in additon to Orkneys and Hebridies, but that would really strain the time table. If only there still was a boat between Lerwick and Bergen (NO), but when you have to go both to and from it just eats too much time.
I tried plotting out the route on a map - does this seem ok? If there are any smaller (slower) roads or more scenic routes, not only would I appriciate a tip, but the other people on the road would surely prefer it if our slow moving vehicle stuck to the country roads instead of the motorway.