As far as I understand it there's no requirement to invoke article 50 at any time unless giving official notice to do and follow the treaty protocols for formal discussions and agreements under a 2 year timeframe.
I`m pretty sure that officially notifying the EU of the UK`s intentions is invoking article 50..
Cameron has made the right decision not to inform them of the UK`s intentions ,but to "steady the ship" till next PM is in place.. The UK is still very much an EU member and must not be bullied or rushed into anything that is not in Britain`s interests...

Merkel has reacted just as the leave campaign suggested she would and I think others will take her stance..
I read that the trade deficit between UK and the EU is about 24 Billion Quid...(Quite incredible)
No wonder Merkel is being so nice!!
I dont think its in their interests to piss us off to much with talks of added tariffs!!
Business is business..Sense will prevail. (I hope)