firstly i would check with haven re taking your van off site. most, if not all sites will charge a fee for this - they own the land so effectively you cannot just get some in to bring a vehicle in to remove it.
normally the site will move it off the pitch, onto a car park etc for collection AFTER you have paid them to disconnect it and move it. there is often a 'gate fee AND conection charges' to be paid at the site you move your van to, so again you need to check out the hidden costs
i reckon there might actually be something in your contract with regards to this. haven tend to have things pretty much sewn up with regards to removing a van off site and a lot of people sell back to them, at considerable loss! with your van being 8 years old, they will tell you that its approaching 10 years old, so dont expect a good price either, but when you consider all the other costs, it might make more financial sense to sell it to them albeit at what might be an insulting price and use your money elsewhere
Ii seem to remember haven offering to allow transfers between their own sites at a much reduced price/deal overall perhaps that might be a question you could ask, you could say you loved haven sites and wanted to move to a different one? though of course their site fees tend to refllect the amenities and location, so if you want a much cheaper site, youre not likely to be where you want to be or for the site to have what you would like it to have.
until you know the process/costs from haven then there is little point doing anything else and you may find it cheaper in the long term to sell it back to them, who will of course resell at a profit!!! or they may themselves sell it off site to a dealer and put a brand new spanker on the plot, more profit that way with two sales and yes they will make a profit on selling to dealer !!!
with regards to site fees, how often you use the van will determine how much value you get from site fees, we pay under 2k per year in france, but only go 2 or 3 times per year (around 10 weeks per year during school holidays, but the van is used by other family members when we dont use it. even at 10 weeks a year (plus travel costs) we think its good value and there is no age restriction and a lot less hassle when selling on than other LARGE COMMERCIAL sites
i have tried to point out SOME of the issues you will need to consider, but you cant do anything until you talk to haven themselves
let us know how you go on, your scenario may help other make an informed decision when considering the intial purchase of a sited static