Softwood is the cheapest of the two varieties of timber due to the fact that trees reach maturity in 25 years or less. Compare this to hardwoods which can take 4 - 5 times longer to grow and you can see a similar increase in prices.
It also helps that the UK is geographically nearer to softwood producing countries so shipping costs would be less. The UK produces a reasonable amount of softwood for its own use. All of these factors mean that softwoods are more readily available and therefore cost less in the UK. IKEA, based in Sweden, use Softwoods and Manufactured Boards for almost all of the furniture it produces, as there are lots of Pine trees in Scandinavia!
Softwoods are used in a huge range of general constructional 'jobs'. In the UK, it is usually Pine (Parana) or Spruce but other types of softwood include Douglas Fir and Western Red Cedar. They are generally cheap and very easy to work with, although some care is required to avoid it splitting when nailing, screwing into, sawing or drilling. It is available to purchase in several forms:
Rough Sawn treated - quite roughly presented - this is used for outdoor use and is pressure treated with preservative to help prevent rotting (Tanalizing).
Used in fencing a lot. You can usually tell that it is treated by it's slightly odd colour, usually a brown or green tinge to it, depending on the colour of preservative.
Rough Sawn - quite roughly presented. It is often used where you won't see it, like 'stud walls' which are covered with plasterboard and are used to divide up rooms in apartments and houses.
Planed - A nice smooth surface used for precision joinery or where you see the surface. e.g. door frames, skirting. Referred to as PSE (Planed Square Edge).