I felt that this book was just back story, or a prequel before going on to the main story, Adam's, in the next book. It wasn't particularly exciting or interesting for me.
A lot of the story centers around Jem and Spider on the run from the police. Which felt a bit silly. If they'd have just gone down to the police station in the first place and explained that Jem had a bad feeling about the London Eye they could have saved a lot of grief. And stopped quite a few people dying in the process.
The idea of a terrorist attack in London was never really touched upon, other than to suggest that the police might think Jem and Spider did it, which they quite clearly didn't.
In the middle of the book I almost lost interest completely and stopped reading it but I wanted to give it a chance. I'm not sure if I go as far as to say it was worth it, as the book just gets sillier from that point but at least I know what happened in it when I read The Chaos.
The last sentence is one to make you want to read the book and one that proves to me that Rachel just wanted to get to the main story but felt she needed to write this so we have some back story. There's plenty of swearing, death and a sex scene in this, even though the main characters are only 15, so it's very British...