OpenData is free mapping layers from Ordnance Survey.
You can order OpenData here. There is no charge (despite having to “order”) – you select an area of interest, they maps you want and then a download link address is emailed to you.
A single area of interest may consist of several tiles, so a download manager may help to transfer the files while you get on with something more interesting.
The following are links to the detailed descriptions of each layer: