According to this
this wikipedia article, there is not MI18, despite there being an MI19. That there is no 13 is easily explained away by superstition, but the idea of a secret organization that should exist but doesn't, is somethat tantalizing. What if they did exist at some point in time (preferably before 1941, since that was when MI19 was created) but later, all evidence was removed.
The question is why, and since magic was dead at that point in time, that could potentially be an explanation. I know I've read somewhere that Churchill consulted a fortune teller quite a few times during WW2, so what if there was a secret occult cadre to match the Nazis? They may have gathered legendary artifacts and such things, and were then dispersed when it was obvious that such things were more or less useless during that era.
That would give the Shadow Court quite a lot of enchantment material, and, considering that Britain was dominant in the Middle East, some really significant relics. It's entirely possible the bodies of several Saints were removed by the MI18 to stop them from being used by the Nazis, for instance, an even some Middle Eastern artifacts rumoured to have magical powers could have been liberated and brought to England. Since it would be very difficult to explain how those artifacts were gathered up without implicating the Churchill administration in some very questionable business, those artifacts probably still gather dust somewhere... where a seer in the Court most certainly would be able to find them after all that latent energy would lead her to search the source out.