Yeah you could get outline planning permission or carry out a pre-app before you buy the land just to see if the idea of a dwelling would be ok, however, the owner of the land will know about it. So although the land could be worth afew 1,000 without planning, once you have that do you really think the owner wouldn't increase their price? All you will end up doing there is paying for something you wont get anything back from...
In some cases it has been known for planners to accept a dwelling on some land to look after animals, but this is rare... and alot has to be done before hand, most of the time they may accept a caravan to be on the site for 5 years before a dwelling can even be thought about... Then you will need to prove that you do need to live there... So don't expect an easy time with this...
Best thing to do would be to look at the Chapter 7 website
http://www.tlio.org.uk/chapter7/index.html (they don't have all the answers, but their website is a useful tool), they sell a planning book for problems like this for 15 pounds, I wouldn't say it was a bible for how to get planning or a loop hole book, but it does explain the law alittle and gives good case studies... So for 15 pounds, its pretty good, and they give free planning advice by phone...
Hope that helps