Back in 2015 we added an OS bench marks layer to the GPS mapping application, originally only for supporters but now for all users. It shows you the original location (they're no longer used and many have since disappeared) of all Ordnance Survey benchmarks.
Last October Lancashire Lad reported that the web site we were relying on for descriptions of the types of bench mark had been taken down. I did speak to the author, John Talbot, about this and he said he's put it back for us but he's not done that so I've now taken his text from the copy at on the Internet Archive and reworked that into our our Bench Marks page which also includes photos of examples of some of the types, taken by us, Lancashire Lad, and Geograph photographers.
The bench mark popup in the GPS mapping application now links to that page.
(Oh yes, and I've also renamed "benchmark" to "bench mark" everywhere as the latter seems to be Ordnance Survey's preferred spelling ... most of the time.)