Half the Books of Blood stories are heavy in symbolism and metaphors (again), almost grotesquely so... Side note, I was a huge fan of King back when I first read them, and really couldn't appreciate how surrealistic it was at times. Growing up I've come to realize those are some of the most clever, elegant and imaginative pieces of horror short stories I ever read. But for instance there is one where two Eastern European cities build two gigantic flesh golems made out of their youngest, most able people, and send them to battle each other. It's about the horrors of war, among other things...
I think Barker paints with very broad brush strokes when he tries to tackle a theme symbolically. He's never really political, it's always about the human condition, so the Illuminati thing really feels too restrictive to be something he'd address, either directly or simply by association...
So, the Midnight Meat train, [SPOILERS !] when it comes to that secret society, seemed more like a symbol of the actual underbelly of large cities, or even about The City as a concept, and how monstrous The City really is, swallowing people to survive and strive... By leaving behind all scruples and morality when it comes to capture the truth, the photographer not only discovers what the truth really is and how fundamentally horrifying it is, but becomes an actual cog in that machine, enabling it to continue its process of dehumanization and assimilation...
But well, since it's never spelled out that bluntly, I imagine your guess is just as good as mine...