Yeah, piss off the entire EU by making them lose big money due to ongoing uncertainty. That's a good start for the UK to get really good conditions in the future negotiations
What is the "uncertainty"? We are leaving the EU, that's that.
Uncertainty is that Cameron and Boris Johnson too, told they are not in hurry to exit at all. Maybe will start to think about it "sometime". And that is f....g prickery in it's nature. If you decided you exit, then exit, don't put all of the EU in uncertain position for a decade. Otherwise it like standing in the door, refusing going out and closing it finally, but instead pissing in the room for owner's appreciation.
My understanding is that making all the arrangements is going to be a very long and complicated process. Perhaps taking many, many years to complete.
Given all the DIFFICULT negotiations, I'm not going to be at all surprised at it taking a long time.
I also understand that (quietly) behind closed doors, the negotiations, discussions and details are already taking place (soon).
E.g. There are various meetings, both in the UK and EU, discussing what is going on, and planned to happen.
Since we are the first BIG EU exiting country (if I understand things correctly, but there was the odd, small thing here and there before, ages ago), it will probably take even longer.