Help

I read a review for a book I'd like to get a few weeks ago, but now I've lost it and can't remember the title!

It was set in England, probably London

It was about some sort of secret government group that deals with the supernatural

The main idea was that there is some sort of monstrous being that the english monarchy made a deal with so it would not destroy the country, sort of like cthulhu or something like that

TIA!