This is a problem, how and who would do that? Three years down the line and it just seems chaos with everyone trying to look after their own agendas. I can't see that changing anytime soon.
Start again. Whilst inside the EU.
Tell them that from 1st January 2020 our fishing grounds are ours, and no-one else's. In return we won't fish in any EU waters. Box 1 ticked.
Tell them that forever, there will be no hard border best Eire and NI. No further discussion required. Box 2 ticked.
Tell them that we will follow their trade agreements (if they allow us) in return we will replicate. Obviously we retain a veto as there will always be exceptions.
If they are not happy with that, then we negotiate the main ones individually. Starting with the ones which are of most interest to the EU, this way if we make concessions then we have a bigger bargaining chip for the ones we want. Box 3 ticked.
So on and so forth. Rather than do it all at once. Do it bit by bit.
Then in 2024, we can say, we've ticked all the toughest boxes, and we'll leave on 1st April. Have a nice day. Most deals are done.
Makes much more sense. And we hold all the cards.