Hello Kev.
This forum is more orientated towards the caravan owner, rather than the sites, but someone may have some input for you so keep reading.
Renting land for caravan storage is considered to be a commercial practice, and you will need to seek planning approval. The authorities in other areas have refused planning for storage sites because the the land is designated as agricultural, green belt, or other infrastructure issues such as inadequate access roads for example could the fire brigade have a good route to the site and loss of amenity or nuisance to local residences. Over capacity of this type of business in a given area. etc.
You will almost certainly need water and drainage at the site even though its just a storage site.
You should also check out the accepted codes of practice for things like the general conditions of land, is it subject to flooding, will it need hardened access roads and storage pitches around the site, and the maximum density of caravans to minimise collateral damage if a neighbouring caravan has a problem, Control of access to the site, who can enter and when, and what they are permitted to do, For example are the caravanners allowed to manoeuvre the caravan themselves? bear in mind if you( the site operator) permit a caravanner to move their own caravan, you can potentially become liable if they damage other caravans on site.
Do you also provide other enhancements such as CCTV or other'security ' measures, and what insurance do you carry to cover claims against the site.
You may find this useful
http://www.fwi.co.uk/business/so-you-want-to-store-caravans.htm
Not in quite the same theme
https://www.pitchup.com/how-start-campsite-caravan-park/