whatever you decide please do lots of research beforehand. many sites have policy about letting the van when you aren't using it, how many weeks you can use it for, whether or not you can sub let it to friends and family, when you cant use it ie. some sites shut. there are 2 big costs to consider. one is the actual costs of keeping your van on the site and the second is that many sites, in order to keep up appearance ask you to change your van every so many years and you have to buy it through them. when you add it up, unless you are going to be spending many many weeks there (ie you are retired) then when you spreadsheet out the cost over say 5 or 10 years and the number of times you will really use it you might find it to be not in the least bit cost effective. good luck.
I am in London so don't want to go right to the end of Cornwall and would like somewhere suitable for older teens, young adults really, (16 -21) with train access so they can go on their own. I have been looking at Par Sands but would welcome thoughts from others who might know Cornwall better than I do as we were a Suffolk holiday family (Southwold) when they were younger and I always went to Wales as a child, hailing from the Midlands.