If you, as Keeper, want to throw a wrench in the players' ordered little lives in Bookhounds or a UK-based, pre-1950s Dracula Dossier mission, why not prosecute them under the Witchcraft Act of 1735?
The Act states that anyone claiming magical powers, or practicing witchcraft, could be imprisoned for a maximum of one year. It remained in force until the 1950s, when it was replaced by the Fraudulent Mediums Act, which in turn was replaced by Consumer Protection under the 2008 Regulations. The 1735 act is the successor to a string of similar acts, all of which had one purpose: to seek out and punish witches, necromancers, and other frauds.
If you look up the Witchcraft Act 1735 you'll notice that the only politician to voice an objection to it was James Erskine, Lord Grange, who is remembered for two things: first, abducting his wife, claiming she died, and holding her prisoner on the Isle of Skye; and second, his opposition to the Act. He opposed the Act because he believed in God, and he, along with John Wesley and others, was reluctant to condemn witchcraft, as it was felt that giving up witchcraft was on par with giving up the Bible. After all, witches were proof of the spirit world, which in turn meant the religious world. Without proof of life after death, where is God? Nebulous claims of Scots political considerations that weighed heavily on Erskine have been floated, but this makes it sound like all of Scotland was witch-ridden, where in practice all of Scotland was merely God-fearing and wanted no truck with an Act that seemed to deny God.
Lest you think this is something that existed but was never enforced, rather like all those Elizabethan statutes that allegedly claim able bodied men have to turn up for archery practice of a Sunday, prosecutions were fairly common up to and after the First World War. This was a time when, if you could bring back the dead for a chat with their grieving relatives, you could make a fortune. During the war the authorities became increasingly concerned that fortune tellers and mystics were having a deleterious effect on morale, particularly since some of them were throwing caution to the wind and making prognostications about the war effort. Telling people who would win the war, or particular battles, was the Government's job, not Miss Millie's, and the authorities took a very dim view of Miss Millie's continued interference.
It got somewhat embarrassing for the plainclothes detectives and undercover agents sent to gather evidence when they discovered that soldiers, even officers, were also going to Miss Millie. Elizabeth Sixsmith, known professionally as 'Madame Betty' - and why she ignored God's gift of a surname I do not know, as Madame Sixsmith to my ears sounds much more impressive than Betty - claimed many soldiers among her clients. In 1918, when Detective Inspector Bedford burst in on medium Marie-Charles to deliver an arrest warrant, he found her in consultation with a Brigadier General.
This activity peaked during the War, when people were at their most vulnerable and desperate for any kind of news. It declined afterward, though the casualty lists ensured there would be a large number of people who wanted to talk to the dead in the 1920s. However as the decades wore on and people became less accustomed to going to fortunetellers and mystics, prosecutions became less common. There's a resurgence during the Second World War, but nothing on the scale of the mischief people were getting up to in the Great War, and once the conflict was over old habits began to die out. There were only a handful of prosecutions under the Fraudulent Mediums Act, and now the whole shebang is shoved in with dodgy tellies and fell-off-the-back-of-a-van stuff you find in the consumer protection regs.
What does this mean for your players?
To begin with, it's an excellent method of harassing them. Are they getting too big for their britches, and pursuing ghouls through Covent Garden without any thought for what might happen next? Time to pull them up and remind them that the outside world isn't something they can just ignore. Sooner or later blatant bullshittery comes back to bite you, and in this case it bites to the tune of one year in chokey. Or perhaps just bound over, if the Keeper's feeling generous. Don't forget that elements of this legislation are still in force in Israel and South Africa to this day, or that it was in force in Northern Ireland long after its repeal and replacement in England in the 1950s. Something for your agents to bear in mind, as they merrily skip across borders accumulating Heat.
Of course, if the government actually has some kind of supernatural or occult-interested research group - say, Edom - then a conviction under the Witchcraft Act could be synonymous with a quick trip to a secluded spot, there to be interrogated at leisure.
As an element of backstory, it can be very useful. Here's Miss Millie, dotty old dear, and here's her gang of hardened thugs. Where did she get hardened thugs? Oh, you can learn a great deal banged up in Holloway for a year. Plus you meet fascinating people like Norah Elam, suffragette and proto-fascist. This works for player characters as well as NPCs, bear in mind, so if you as player want to justify that point in Streetwise and some of your less savory contacts, this would be a good way to do it.
On top of all that, there's one other obvious direction to take: the prison scenario. This last is very much up to the Keeper, but consider: if you want a one-off in an unusual setting, and think a bit of hard time could be an interesting route to take the campaign, the Witchcraft Act will get you there in style. I recommend a good old-fashioned haunting, with the execution of Edith Thompson - and possibly also her unborn child - as a catalyst, but really, with a prison that's been there since the mid-nineteenth century, you could do anything.