You are 180 degrees out of phase.
The big initial problem with a no deal is that nothing, and no one, will be able to get into the country (at least not as simply as can be done now.)
It is not about being invaded
Foodstuffs will be stuck at the borders; (I could be wrong but I don’t think) any insulin is manufactured in the U.K., ... we live in a just in time economy and a no deal Brexit will cause that to cease to work in, err, time.
On a minor, but personal note, my 50th birthday is (Thursday) 28th March 2019, I already have the following week off. For the last 18 months or so, my wife and I have decided that we would have a mini break in, perhaps, Croatia or Berlin to celebrate my big day.
If we crash out of the EU, we do so on Friday 29th March. I have deliberately put my plans on hold, and have not yet booked anything. I cannot be sure that flights will be flying (airlines & pilots will no longer be insured & licensed) I suspect that businesses- with a far greater economic clout than little old me - have been making (or more appropriately, postponing) plans too.
Just to reiterate - nobody other than leavers have ever been worried about anyone “invading” (although interesting that “leavers” use the language of invasion)