The arc is scattered, the issues collected in the graphic novel are as much establishing the back story as they are dealing with what's happening now. Even so the details of what the 'Injection' actually is are thin on the ground, as are the ways that it manifests - I assume that anything out of mythology is fair game and readers might see Black Annis or parts of the Mabinogion come to life (though I would bet against a romantic version of King Arthur because the nature of the book feels more primal than that).
Pulling our focus back, let's look at the world Ellis and Shalvey are detailing - on the surface it's pretty normal, but there are hints of weirdness even in the midst of government, Westminster and White Hall have 'the Department of Time and Measurement - presumably analogous to the Department of Weights and Measures only more sinister - which administers something called The Breaker's Yard. Given that 'not Merlin' is the character connected to this place, It's a 'ghost hunting' or 'monster killing' outfit, which in turn begs the question of whether modern Britain is something created by magic, and the 'Injection' is simply bringing those elements back into play.
Pure conjecture on my part, but the book opens up questions for me, ones that I'm looking forward to Ellis answering as the series progresses.
The book feels like Ellis is meditating on the nature of Englishness, all that 'matter of Britain' stuff, and the mythology that we have largely forgotten but which informs the way our nation is today. It's something that seems to be picking up over in Trees, where he channels his inner John Wyndham on a global scale. While I would say that he isn't promoting the sort of flag-waving, animalistic, patriotism we've seen emerge in Britain over the past few months (though 'break the surface' would probably be more accurate - it's always been there but for the most part it was hidden), this is much more thoughtful, a meditation on the roots of our identity as a nation. I quite like it, but I can see that a lot of people would not.
I would recommend this if you like Ellis' other work and if you enjoy Strange England type stories.