MI5 are able to identify these people.
If they are, and if they can actually tag them to a crime, then they will arrest them. If they don't it may be because they are hoping to follow them to bigger fish.
Fact of the matter is verbally supporting ISIS, or calling for Shariah law, is not a crime anymore than Neo-Nazi's supporting Hitler and calling for white rights is. At least not until they can be determined to be inciting violence. Which is why hate speech laws are so important. However, you can't arrest these guys for verbally supporting ISIS and not arrest KH for calling for genocide.
Either we uphold our values which include a right to trial, freedom of speech and freedom of religion and movement or we toss them away, we can't pick and choose who we toss them away for. We can't say it's ok to be a white supremacist but not ok to be an Islamic fundamentalist.
The downside of a society that supports human rights is that sometimes horrible things like this happen. It's the danger we face by being an open society. And either you support an open society and accept that danger as part of our freedoms, or you call for a society that values "safety" over freedom and human rights, in which case I suggest you move to ISIS territory, because they want the same thing.