SO the main cost is the land. Land is expensive in the UK and you need to buy land with planning permission or you could end up with a wasted investment if you do not get planning permission on your land. Prices vary dramatically aroudn the country, ranging from hundreds of thousands in London and the South East, to thousands in Wales, Scotland, and other areas further from the capital.
A good palce to look for building plots is http://www.ebuild.co.uk/building-plots.htm
Next is the size and construction of the house. Obviously a unique designed house with some high-specification features will add cost. A traditional brick built house of a fairly standard design will cost a lot less, although may not be too satisfying to live in if you have had the opportunity to stamp your taste into its fabric!
http://www.ebuild.co.uk/
One guide is the rebuild cost used in house insurance. For instance a £500,000 house in London has a total rebuild cost of £190,000 so that may give you an idea of the value of the land and the savings to be made if you build your own home.
If you ever get a chance, watch the Channel 4 series Grand Designs. Its great! All about people who build their own homes. One common theme: hard work and over budget!
http://www.channel4.com/4homes/ontv/grand-designs/