Use of language
Guidelines on the use of language when reporting on extreme violence in pursuit of political or ideological objectives can be found here.
When talking about IS, Editorial Policy advice is that the term Islamic State should be qualified eg. “Islamic State group” or “so called Islamic State”, or “self-style Islamic State”, or “Islamic State fighter, militants, extremists” etc. and then as IS in the rest of the bulletin/body of text. Islamist should be used as the adjective to describe extremism linked to groups like this and not Islamic or Islam.