English law has only partly transitioned to public prosecutions. Private prosecutions are still legally the norm, but CPS prosecuting a case is de-facto the norm, as they have the right to take over a private prosecution. In most cases, CPS either takes over a case and prosecutes it, or takes over a case and drops the charges (if evidence is wholly insufficient, or continuing prosecution is against public policy). But this leaves the awkward vestigial category: cases that CPS chooses neither to intervene to prosecute nor to intervene to drop, which can go forward at a private party's own expense.

(Note: Only true of English/Welsh law; Scots law makes it much harder to bring a private prosecution.)

Thanks for the info, a quick google showing the rarity of private prosecutions in Scotland is definitely encouraging!

Ah but in an ironic twist, I'm sure some film maker will create a movie about this. Oh wait, I think they have...