The Muslim leaders and scholars have repeatedly spoke out very loudly and clearly against ISIS and terrorism. The problem is the media never picks it up, so you and I think they are not talking loud enough, when in fact they truly are and tbh probably fed up of constantly having to repeat themselves.
Nothing ever justifies joining or promoting the extremist ideology of ISIS. However, there is a deeper problem here and that is the government needs to engage more with those that appose parts of its foreign policy. There is also a huge issue of anti-Muslim hate and sentiment in the media, which is making some Muslims feel utterly isolated and like they don't matter at all (and there is some truth to that!)
Just recently, I heard quite a few Muslims saying that it's strange how when a non-Muslim kills over a 100 people in an air crash, that is suicide, but when a Muslim does that (who has mental health problems too) it's terrorism. Like it or not, there are some double standards when it comes to the way some Muslims are treated in the media and by the government (think about the blatant turning an eye against Assad's crime or Israeli crimes). What's great about British values is equality and justice, and we need to extend to the Muslim community too. I'm not saying that this justifies the extremist actions, far from it, but if we want to get the root of the problem, those issues need addressing too.