Mileposts vary in design and size. Older ones tend to be of whatever style the owning railway company decided upon. Some have different shapes for each incremental post, some different shaped heads. Some will only show the mileage on whole mileposts, with spots, stripes, numbers, fractions or bits sticking out of the top to indicate the quarter mileposts.
Modern posts tend to follow broadly the same styles, with the whole mileage over the top with the chainage underneath.
South of London extending into the suburbs on many lines, there are actually chainage discs every chain counting from one to eighty, any remaining mileposts sit at the 0, 20, 40 and 60 discs.