I should also mention: many Ordnance Survey maps are not suitable for use as data sources for OSM. Only the OS OpenData StreetView and OS OpenMap Local ones have a compatible license. They are available as layers in the ID Editor (see the background panel on the right).

In particular, OS 1:25k and 1:50k mapping is not permissible as a source for OSM, as the license is incompatible.

See https://wiki.openstreetmap.org/wiki/Ordnance_Survey#Map_licence for more information.