Various web sites that may be of use for further study into Bench Marks
https://maps.nls.uk/ the National Library of Scotland has has a project to digitalise their collection of maps, and as the NLS is one of the National Depository Libraries they hold a very extensive collection, which is now very useful for searching OS mapping (and others) and are accessible on line and can be searched and zoomed in to discover BMs in greater detail at various scales and dates etc, maps can also be printed or copies ordered.
https://www.ordnancesurvey.co.uk/benchmarks/ the OS keep information on BMs that can be searched via your grid ref square as well as historic information.
https://interactivemaps.uk/os-benchmark-archive/ one of the latest products from OS which consists of a zoomable map that you can move to your location and zoom in as well as put up a grid with grid square shown, the BMs are indicated and when zoomed in more details are shown such as height.
https://www.bench-marks.org.uk/ the bench mark database that you can access and/or join to be able to log your BM finds as well as search the database in your location by post code or grid reference.
https://historicengland.org.uk/listing/what-is-designation/ general information on listing of buildings etc. not of specific use to BMs but of interest.
https://sheetfinder.charlesclosesociety.org/ society for people interested in the study of OS maps with newsletters and information on local meetings. Useful for general mapping information with many articles written by members on many aspects of mapping.
https://www.stalbanshistory.org/ my local history society with many talks, newsletters and a local history library with a good collection of local (St Albans) maps at a scale suitable for finding bench marks.
https://www.milestonesociety.co.uk/ for people interested in mile stones, many of which have bench marks on!
https://www.geocaching.com/mark/ I have not used this site so can only say that it may be of general interest?
https://www.newlynarchive.org.uk/ a local archive dedicated to the study of Newlyn Cornwall which includes local history publications including the Tidal Observatory/National Datum.
https://www.ntslf.org/tgi/newlyn-tidal-observatory specialist website with interest in the Tidal Observatory.
http://www.alangodfreymaps.co.uk/ reproduction of OS maps from the various 1:2500 scales (also 1″ but these don’t show BMs!) over 3500 maps reproduced covering popular areas around the UK.
www.nationalarchives.gov.uk/ a good place to find information on OS historical documents, you could find items needed by pre-booking items that you want to view from their collection but it would be best to register as a reader prior to your visit.
https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/maps-further-research/ detailed guidance to the national archive on maps and mapping.
https://www.cartography.org.uk/ another website that I hope to spend time getting to know but should be of some use?
http://www.cassinimaps.co.uk/default.asp list of interesting maps but mainly smaller scale (1″ etc) which don’t show BMs!
https://www.bl.uk/ one of the depository libraries so holds a collection of most maps but you need to visit to view the collection (unlike the National Library of Scotland see above which has digitalised their map collection and made it available on line as well as for purchase).
https://Trigpointing.UK/ mainly concerned with triangulation pillars and bagging them (logging) but still has some useful information on bench marks.