You do not need to download all of a map to use it. The maps are broken into smaller areas, which when downloaded seamlessly join up. You can also use Map Streaming.

OS 1:50,000                  3.5 GB

OS 1:50,000 HD            8.0 GB

OS 1:25,000                  8.0 GB

OS 1:25,000 HD            28.0 GB

OS Vector Map Local    34.0 GB