Dave did not evoke Article 50 yet because it makes sense to move slowly, letting things settle, and work out exactly how to proceed.
Remember, with the exception of Greenland, we are into new territory here. No one else has had the guts to tell the EU, "Enough".
Juncker is simply mischief making when he pretends it makes no sense not to go immediately and cause maximum turmoil and market uncertainty in an effort to cause as much turmoil in the world's economies as possible. He's rapidly losing whatever credibility he still held.
If this, or is an election occurs first, the next, government didn't proceed with withdrawal, I'd predict a riot, in fact multiple ones. Unless someone insane got elected and appointed, no PM would even consider it.