IMO they are charging him with perversion of justice because they are not confident that they can prove kidnapping or murder. They feel a need to charge him with SOMETHING. As in the article posted this morning the "key evidence" is the testimony of a seven year old. A seven year old who said she saw April get into a light colored/white van not a dark blue landrover. Who wants to bet this wont be the same testimony we hear at the actual trial?
If they can't prove he is guilty of child abduction or murder, I don't see how they could then convict him with unlawful disposal and concealment of a body? Here is the full charge
That between September 30, 2012 and October 3, 2012 at Machynlleth, with intent to pervert the course of justice, did an act, namely the unlawful disposal and concealment of the body of April Sue-Lyn Jones, which had a tendency to pervert the course of public justice contrary to common law.
http://www.telegraph.co.uk/news/uknews/crime/9594065/Mark-Bridger-The-charges-against-him.html