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. We do not use the name Da'esh for IS but we explain it when others use it.
Guidelines on use of language can be found in sections 11.3.5 and 11.3.6 of the guidelines.