Let me just say... the other user is wrong in saying it's sarcastic. The BBC aren't being sarcastic when they say "so-called Islamic state". I'm a English speaker (I'm English) and I am confused by the other user saying that to be honest.
Context: David Cameron said in parliament that we should call ISIS Deash so that people do not associate ISIS with Islam and therefore all other Muslims. The BBC basically said "well, we've been calling them Islamic state for so long we can't start calling them Deash because it could confuse people". Now they call them "so-called".
Source to verify I'm not making it up:
I personally think that using the term Isil or ‘so-called’ would be better than what [the BBC] currently do,” Cameron said. “I don’t think we’ll move them all the way to Daesh, so I think saying Isil is probably better than Islamic State because it is neither, in my view, Islamic or a state.