There are residential parks, but very often, the units are almost as expensive as a house. Coupled with that, there is generally an age limit such as over 55's only, no children living on site etc.

There are plenty of holiday ones where you live for 9/10/11 months, but have to be living somewhere else for the rest of the time. That is great if you don't have children - schooling problems.

I have been searching for a long time, but not found anything suitable for a family where the costs aren't the same as the cost of a house.

Some sites to look at
http://www.ukparks.com/residentialhome.asp?sltParkType=parkhomes

http://www.parkhome-living.co.uk/nphc/index.php

http://directories.thecaravan.net/residentialparks/

Like everybody says - read the smallprint very carefully and really make sure you know what you are taking on.