Dave,
Cameron knew very well that the referendum was advisory. If he did not he would surely be the most ill informed prime minister in history, which I do not believe.

The legal case on whether it was advice or a decision is not yet finished, so Cameron might yet prove to be right.

And it was not just Cameron.

Corbyn,
"The referendum has taken place, a decision has been made, I think we have got to respect that decision and work out our relationship with Europe in the future."
("Decision" not "advice.")

The Independent 23 June.
"For now, both sides are, officially at least, working on the assumption that the British people's decision on 23 June will indeed be final."

http://www.independent.co.uk/news/uk/politics/what-is-brexit-why-is-there-an-eu-referendum-a7042791.html