I'm not sure what they think is available for discussion. The type of exit is clear, it's the one where a country triggers Article 50 and then awaits the EU to getting around to oking it.
All the rest isn't about exiting it's about the relationship and agreements post exit.
No one knows what can be negotiated/agreed around the table for that, for we can not be sure how vindictive the EU can be to a resigning member of their control group. One would hope they'd go for a win/win agreement (which would benefit them more than us) but one can not tell. Presently they are still mainly continuing with threats they hope will eventually stop the process of returning our sovereignty to us.
In any case what do we threaten to do if we don't like what they come up with ? Threaten to stay and thus give them, and those who wish to deny the will of the nation, what they voted for ? It's a nonsense.
All that needs to be clarified before the government is allowed to get on with it is confirmation is that the return of out nation's sovereignty is not open to negotiation, which includes total border control (so 'free movement' is gone), and beyond that, those at the table are already out to get the best possible deal for all anyway so further input isn't helpful. Before that, there is no more detail that can be given since during negotiations details change as compromises are made, anyway and any stated aims may well be dropped.
As for putting the end agreement to the public or the House for that matter, that may be dressed up to sound reasonable but it is effectively the old EU trick of saying, "You gave the wrong result last time, we don't approve, try to vote differently this time". The hope being that few would change from 'remain' to 'leave' simply because they knew the decision had already been made and so ethically should stand, but would relish the chance of trying to stop it again: whilst there will be a proportion that had the courage to vote for change last time, but since folk don't like change may well wimp out having been scared by remain fear tales, if asked again. If is a clear attempt to derail the people's decision, and would leave folk in no doubt that public opinion is simply not valued by the elite. If elites spread enough fear they should find the people vote as they are told to.