Back on the "crashing out" point, I agree that this is what leaving without a deal means in practice.
Take the "planes won't be able to take off or land" threat, for example. The agreements which currently allow planes to take off and land in the UK is linked to our EU membership. If we leave with no deal and no interim arrangements to cover air travel, then this is literally what will happen. Probably (hopefully) only for a short time, but even that would have a huge financial cost.
In France the SNCF had a series of train strikes between April and June where staff went on strike two days out of every five. At the beginning it caused genuine chaos. I ended up working from other locations two days a week for several weeks because getting to the office was just too much of an arse ache. There were trains running on my route, but at the beginning of the strikes there were only a few during rush hours and none between about 10am and 5pm. After a month or so they had managed to resume a more-or-less normal service, just with lower overall capacity and fewer trains running. By the end there wasn't really a noticeable difference between strike days and non strike days.
These strikes are estimated to have cost the French economy around 800 million euro.
The UK operates over 60 airports and handles over 8,000 flights per day.
What is going to be the total cost to the UK economy if we are unable to operate any flights even for a single day? A week? A month?
Pretty huge.
For a general idea, check out this old BBC article attempting to quantify the economic impact of the volcanic ash cloud which disrupted European air travel for six days in 2010.
news.bbc.co.uk/2/hi/8629623.stm