In reply to purplemonkeyelephant:
> Google has free maps for the entire world and I can't get a proprietary map for a £250 GPS.
Does your GPS give you adverts? Does it collect information on you it can sell to advertisers?
Different products, different business models, I'm afraid...
There are many open-source mapping projects, based on processing the SRTM DEM data to create a 3D topographic base map, and then crowd-sourcing the addition of man-made and natural features. e.g. OpenStreetMap and its offspring.
Data gathering for mapping is expensive. So you either pay for it from a traditional mapping agency (e.g. OS), get it crowd sourced (OSM), or get a multi-billion-dollar advertising company to pay for it (Google), who will use it to further increase their revenue by selling location-based services.
Problems with more complex mapping GPS receivers are that their screens don't work well in low temperature, and you may have trouble reading them in sunlight. More basic models with simple alphanumeric, dot matrix LCDs can work at lower temperatures, and can be transflective, so illuminated either by a backlight, or by sunlight.