Under the process set out in the Lisbon Treaty, a nation has two years to complete a deal once it formally declares that it will withdraw from the EU.
So what you seem to be saying is that the UK should treat all its commitments and treaties as not worth the paper they are written on. That's not gonna do a lot for any deals it wants to make (with any country) in the future is it? "Yeah, sure you'll sign the paper, but we can't trust you to keep your word, so forget it".