The legal complexities of the UK's exiting the EU. Despite the PM's pronouncements, Parliament must have its say - or we never had any control to "take back" in the first place. 2. Progress on some sort of free trade agreement with the EU. This will obviously be embryonic and principle-led at this stage. Which means there must, surely, be... 3. Some sort of interstitial agreement for the period following the invocation of Article 50. Expert EU lawyers are unsure as to whether or not the UK can unilaterally un-invoke it, irrespective of what the other 27 member sates say. 4. Negotiations with the World Trade Organisation - the UK will have to get approval of the terms from over 160 countries. Unanimous approval. 5. Progress on the individual negotiation of trade treaties with 53 countries with which the EU has an existing deal. Before Britain can even begin to contemplate leaving the EU all this must be in place.
Aside from the alarming issue of whether Leave voters really knew what they were voting for, there's also a huge "To Do" list of purely administrative chores (again, the BBC offers a briefing).