I guess we'll know by mid December if 'No Deal'. In that case, I predict a load of temporary deals will be struck to cover key regulation and trade areas. Going forward, these will mean rolling crisis periods as they repeatedly expire & have to be renegotiated & some groups fluster "We can't go on like this forever!"
Another problem is all the minor areas nobody thinks to strike a deal for, in advance. Those will be overlooked with sudden weird unexpected ripple effects of crises going on as problems manifest & very temporary solutions get hastily patched together. Repeatedly.
Also, our industry & shops will move from 'just in time' mode to having lots more stockpile. This will mean waste sometimes, shortages sometimes. Shops suddenly completely out of things or prices spiking. A whole new industry of 'storage' services for goods & parts will spring up. Logistics will Boom.
I must admit I am very intrigued when HappyBrexiters say everything will be fine, because " It is in the interests of all participants." It was in best interests of all participants to not Brexit at all... yet here we are.
Plan B, I reckon, is revoke A50 in January but with politicians promising to keep negotiating for actual deal and then re-invoking A50 when the deal is figured out (when the technology exists... when the rainbow fairy kitten unicorn flies...)