In a sense, David Cameron's position on Islam is to be expected. As British Prime Minister, he is of course too busy to study Islam in detail. And even before becoming PM (in 2010), my guess is that he would have spent almost zero time studying Islam. Sure, he would have read about Islam, Islamic terrorism, and Islamism in the news. Nonetheless, I doubt that he gave such things much thought when out of power. He would have been far too busy planning his rise to power and, in a auxiliary manner, thinking about tax issues, the NHS, the structure of the Tory Party, competing with the Labour Party and whatnot.
It is of course true that Cameron should have studied Islam independently from the Conservative Muslim Forum, Baroness Sayeeda Warsi (the unelected Minister for Faith and Communities) and his advisers (i.e., the higher civil servants). After all, he was 35 years old when 9/11 occurred and 39 when 7/7 occurred in the city of London. He will now also know about the massive Muslim grooming of young non-Muslim girls in the UK. He knows about the 85 or more sharia courts in his country, the weekly political activism of Islamists on the streets of the UK, the 15 foiled Islamic terrorist attacks between only 2005 and 2008, etc. He also knows about the the Taliban, the Islamic civil wars in Iraq and Syria. He may even know about the Islamist slaughter of over one and a half million Sudanese Christians and black animists in the 1990s and 2000s.