This one worked for me:
OSGB 1936 / British National Grid EPSG:2770
To answer your questions:
"Can you advise me something about finding CRS in general?"
You would start with looking at the age and check which map projection is usually used within a country during that time in https://epsg.org/home.html Given a Pre-WWII age in Britain doesn't ...
Try this command from your GDAL shell:
projinfo -s epsg:4326 -t epsg:27700
You will probably see at the end of the output
Grid OSTN15_NTv2_OSGBtoETRS.gsb needed but not found on the system.
Can be obtained from the proj-datumgrid-europe package at
Download the grid and place it into the ...
A second approach is to look for the "Grids and Datums" columns of Clifford Mugnier where he highlights the mapping history of a country.
This is his version for the UK:
I think it should be easy to find the elevation data. After you have found this (as a raster tif file) you can calculate the slopes in for example QGIS. This is done by just clicking a few times so this should be fairly easy.
It sounds like you need my experimental QGis plugin which gives you the gridSquare function to use in labels etc. The first argument controls how many digits you get from 100 giving TF to .001 giving TF3928703326.