The OS and Met Office (and various other bodies) are established as Trading Funds. That means that they are responsible for their income and expenditure and hav

The OS and Met Office (and various other bodies) are established as Trading Funds. That means that they are responsible for their income and expenditure and have to balance their own books - the government pays for services in the same way that the private sector does. If the OS didn't sell its data, the government would have to pay a lot more to have the range and quality of cartographic services that are available. It's also worth saying that government departments are free to purchase mapping (or weather) services from other agencies in the free market, and I've seen OSM based providers winning contracts before. Finally, weather data is made freely available by the Met Office, but it is the forecasts are commercial. The same base data is used by eg Meteo Group for their forecasts.