Only just found this thread.
Around the turn of the century, Yale began exporting a small-case cylinder rim latch to Britain. Several British makers copied this, as the early patents had expired. The Yale-type copies were interchangeable with both the Yale cylinder and lockcase.

Further to compete with the Yale latch, several British makers produced small-case lever cylinder rim latches with lever cylinder locking devices. None of these was compatible with Yale, or the other British makers, Hobbs, Gibbons, and later, Albert Marston (Wellington).
The op's latch appeared early in the 20C but I have not found the patent, though there are some dated adverts for it. Mine is from a building of 1912, and the one on the front door of Polesden Lacey house (NT) is from a similar period. Mine has a pin key, a square connecting bar, and the same type of latch case.
The cylinder was made with both pin and pipe key variants, and the cylinder markings and shape varied slightly during the course of production. Several variations of latch case are also seen.
Afaik it was made through the 1930s. But only the Wellingon latch achieved much success in the marketplace, and that nowhere near matched Yale.
I have seen two others in service, and several on display in lock shops.