The Bench Marks are shown on 1:1250, 2500 and 10,000 OS plans along with their value, they were originally located roughly every 1/4mile in built up areas. The OS has a list of them but no longer maintains or install them, the coming of digital mapping, total stations and differential GPS has taken over. I used to use them a lot when surveying, it was frustrating to go on site and find the BM you were planning to use has been removed!
The bronze plate ones are fundamental BMs and are more widely spaced, they are designed to have a bracket inserted so you can rest the staff on it. On the incised ones, the staff holder uses something like a credit card to get the staff on the horizontal bar.
I can thoroughly recommend Map of a Nation by Rachel Hewitt if you want to learn more about the OS.