Or indeed in one notable case the person who was arrested for a T-Shirt about "Plasticine action"
Whether or not the proscription was correct is irrelevant, the current law means that you commit the same offence showing support for IS or the Terrorgram Collective.
The police can’t simply ignore one proscribed group over another as that leads to all manner of weird and wacky outcomes.
Having a law that means merely expressing support of a group, leads to criminal charges is not something I think should be in place in any country that pretends to support freedom of speech.