I am not sure what exactly you are asking.
Some of the maps on offer by the NLS can or could be accessed via the TransDEM map tile client. However, that changes over time and for some, access is no longer possible.
The 1:25000 from the 1950s still seems to work, so do the very large scale 25 inch maps from around 1900. 6 inch apparently has changed the server and now uses a key. I don't know whether that key is session based or permanent. So 6 inch may work or it may not.
This is the setting for the 1:25000 and for the 6 inch where 1:25000 is still the same as in the 2018 sample settings.
Code:
<?xml version="1.0" encoding="iso-8859-1" ?>
<TileServer version="1.0" >
<Provider name="8 Historic UK" guid="8eecd61c-4999-49dd-b69c-771769533686" curName="Historic UK" >
<URL name="25000" guid="ff5ade01-4859-4def-a11b-203147d4d423" addr="http://www.sabre-roads.org.uk" prefix="maps/tile.php?l=os1" standalone="1" type="4" >
<Matrix order="2" prefixClm="&x=" prefixRow="&y=" prefixZoom="&z=" minZoom="6" maxZoom="16" />
<Mercator origLat="90" origLon="-180" signLat="-1" signLon="1" projection="4" />
</URL>
<URL name="6 inch" guid="22abb764-2392-4fa6-91b4-63702b727f89" addr="https://api.maptiler.com" prefix="tiles/uk-osgb10k1888/" standalone="1" type="6" postfix=".jpg?key=7Y0Q1ck46BnB8cXXXg8X" >
<Matrix order="2" prefixClm="/" prefixRow="/" minZoom="1" maxZoom="17" />
<Mercator origLat="90" origLon="-180" signLat="-1" signLon="1" projection="4" />
</URL>
</Provider>
</TileServer>
As always, copy/paste to a text editor, save as xml file and import with the TransDEM map tile client.
For the 25inch maps see here:
viewtopic.php?p=3082#p3082