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