Bumpkins
Sorry, but you're not the first person to think of this. For a Local Authority to permit the development of a permanent residential unit, it must be fully compliant with the building and planning regulations / legislation - which a mobile home is not (by a long shot). It goes beyond site servicing; it is also about room sizes, construction materials, life of the structure etc, etc, etc.
As threebedsemi and onq said, you can now buy prefabricated wooden/bailed etc houses / cabins (as well as seomra's) and of course traditional block houses for relatively low costs, which will get through the buildings regs and planning processes. However, you have to bear in mind all the costs that come with this: site servicing, build, contributions, conditions, etc etc.
I know of cases where people simply take the risks of doing something like you propose and if the Local Authority find out and force them to remove the offending development they simply forego the losses and comply.
In short, don't waste you time and money, or else make sure you are aware of all the potential problems and costs!