I looked in to doing exactly this a few years ago and was put off by the horror stories.
A mobile is a depreciating asset and site owners can have horrendous terms and conditions- when the home reaches a certain age it has to be replaced; you can't sell to anybody but the site owner; the site owner fixes the prices of utilities; fees go up above inflation...
I was shocked at how expensive they can be:
http://www.houseladder.co.uk/SearchEngine/Prop ertyDetailsForSale.aspx?PId=256773
The sites can have rules which you may find more than petty annoyance - when you can hang your washing out (if you can) for example. And committees to make and enforce the rules.
And lack of space. Fine if you are happy to get rid of a lot of your personal possessions - the stuff in your spare bedrooms, loft and shed.
Maybe things are much better now, but there have been some horror stories on quite recent 'Watchdog' type programmes.