Yes, I admit I was surprised too, though the BBFC are very lenient these days about the imagery that can be included in a 15 (I think the proviso is there cannot be dwelling on sadism, but it can be fairly gory).

Personally I liked the gruesome, surreal, bonkers nature of the final act, but then I'm a horror fan. My review is here, if you're interested.

Great article by the way. :)