As I repeatedly explained before, there is absolutely nothing to gain for Britain from leaving the EU.
At best they can strike a similar deal as Norway or Switzerland have. But the British government seems to think they can keep all the goodies while getting rid of everything they don't want. I'm not sure yet if that is a loss of reality, simple stupidity or tactics in order to present themselves as victims of the EU when the negotiations fail. Maybe a bit of everything.
The EU is forced to make it as hart and painful for Britain as possible - not because they're spiteful or vindictive but to keep the rest pf the EU together: What signal would it send to other member states of you can have all the niceties but can get rid of the inconvenient stuff? If Britain wants to play hard, they will run into a very hard wall.
Don't forget that any result of the negotiations has to be ratified by
all of the 28 member states. If only one member state says no, because they think Britain gets more than they deserve, the whole deal is off.
Knowing the way EU negotiations went in the past, there is no chance in hell they will make it within the given two years.
The whole process is incredible complex and has many side-effects people will not think of right now. For instance today I read this:
Future of Gibraltar at stake in Brexit negotiations