Georeferencing is the process of registering the raster images within a geographic framework by assigning map coordinates to the image data and resampling the pixels of the image to conform to the map projection grid.
The 1:250 000 Scale Colour Raster supply is not georeferenced in any way. This means that when you load the tiles into a GIS, they will not be set up in geographic relationship to each other.
To view the map tiles in correct geographic relation to both the National Grid and the other tiles, you need to georeference the map tile files. Most geographic information systems (GIS) provide georeferencing functionality, but for each set of tiles you need to provide information on how the tiles should be ordered.
Ordnance Survey provides this information in a set of georeferencing files for 1:250 000 Scale Colour Raster. These files contain the National Grid corner coordinates for each 100km by 100km tile. You can download the files for free on the Georeferencing files and land and sea tiles page of the OS website. Please check which format your GIS supports before downloading. The TIFF World Files (TFW) are for use in ESRI® ArcGIS and the TAB files for use in MapInfo®. You should save the georeferencing files to the same directory as the map tile files to ensure that they work correctly. Great Britain is surveyed and mapped using the Transverse Mercator (or Gauss-Kruger) projection, so all raster tiles will be mapped to this projection (as it applies to the Ordnance Survey National Grid) when using the georeferencing files.
Some available world files contain only sea, and for this reason, no data is available. These files are included to allow you to complete the coastline.