There are now many more large-scale OS maps available on the NLS website. They are on the same terms (CC-BY) as the maps we are already using. The ‘new’ maps cover much of Great Britain, especially built-up areas. They include newer editions of maps we are already using.
I have added some of the maps to the list of layers in JOSM. They can also be added manually in iD (all tms with a zoom range of 1-20):
NLS - OS 1:1,250 National Grid Maps Ed. A SH - TL
https://geo.nls.uk/mapdata3/os/1250_A_3/{z}/{x}/{y}.png
NLS - OS 1:1,250 National Grid Maps Ed. A TL - TR
https://geo.nls.uk/mapdata2/os/1250_A_4/{z}/{x}/{y}.png
NLS - OS 1:1,250 National Grid Maps Ed. B SJ - TR
https://geo.nls.uk/maps/os/1250_B_1eng/{z}/{x}/{y}.png
NLS - OS 1:1,250 National Grid Maps Ed. C
https://geo.nls.uk/maps/os/1250_C/{z}/{x}/{y}.png
I have also updated the wiki National Library of Scotland - OpenStreetMap Wiki
You can find more layers on the NLS web site. Go to the Georeferenced Map viewer https://maps.nls.uk/geo/explore/ and find the map you want. Click on Overlay at the bottom of the page. Check that it says Re-use: CC-BY (NLS). Then click on the link for the map layer. This will open a how-to-use page (although there are some links which don’t work). Click on XYZ and a URL will appear in a box just below. Copy the URL and add it manually to JOSM or iD. Here is a list of all the new URLs not mentioned above.
1:1250 scale, all tms with a zoom range of 1-20
Scotland, edition A
https://mapseries-tilesets.s3.amazonaws.com/os/1250_1967/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/1250_1968/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/1250_1969/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/1250_1970/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/1250_1971/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/1250_A_1972-4/{z}/{x}/{y}.png
England and Wales, edition A
https://geo.nls.uk/mapdata3/os/1250_A_1/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/1250_A_2/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/1250_A_1974/{z}/{x}/{y}.png
Great Britain, edition A
https://geo.nls.uk/mapdata2/os/1250_A_1975/{z}/{x}/{y}.png
Scotland, edition B
https://mapseries-tilesets.s3.amazonaws.com/os/1250_B/{z}/{x}/{y}.png
Great Britain, edition B
https://geo.nls.uk/mapdata2/os/1250_B_1973-5/{z}/{x}/{y}.png
Scotland, edition C
https://mapseries-tilesets.s3.amazonaws.com/os/1250_C/{z}/{x}/{y}.png
Great Britain, edition C
https://geo.nls.uk/mapdata2/os/1250_C_1973-5/{z}/{x}/{y}.png
1:2500 scale, all tms with a zoom range of 1-18
Scotland, edition A
https://geo.nls.uk/mapdata3/os/2500_1967/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/2500_1968/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/2500_1969/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_1970/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_1971/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/2500_1972/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_1973/{z}/{x}/{y}.png
England and Wales double sheets, edition A
https://geo.nls.uk/mapdata2/os/2500_A_1D/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_2D/{z}/{x}/{y}.png
https://geo.nls.uk/maps/os/2500_A_3D/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_4D/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_5D/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_6D/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_7D/{z}/{x}/{y}.png
England and Wales single sheets, edition A
https://geo.nls.uk/maps/os/2500_A_1S/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_2S/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_3S/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata2/os/2500_A_4S/{z}/{x}/{y}.png
https://mapseries-tilesets.s3.amazonaws.com/os/london_2500/{z}/{x}/{y}.png
Great Britain, edition A
https://geo.nls.uk/mapdata3/os/2500_1974/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/2500_A_1975/{z}/{x}/{y}.png
Great Britain, edition B
https://geo.nls.uk/mapdata2/os/2500_B/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/2500_B_1973-5/{z}/{x}/{y}.png
Great Britain, edition C
https://geo.nls.uk/mapdata2/os/2500_C/{z}/{x}/{y}.png
https://geo.nls.uk/mapdata3/os/2500_C_1973-5/{z}/{x}/{y}.png