It is always a terrible investment.
You are actually buying an obligation to pay annual ground rent subject to increases and other charges, and there is usually a requirement to upgrade your unit after x number of years which despite flannel about very attractive deals, will cost you a fortune. Your rental return will be limited by the British season and the site will usually take a large cut. There will be strict rules regarding renting out your unit and you may not be permitted to do so on some sites.
If the site for bust or is sold to another owner, you may find yourself in an undercapitalised, scruffy site with no recourse to compensation.
Like a car but more so, the minute you buy the static is starts to depreciate, unlike a freehold house you don’t own the land.
For some people, the pleasure it brings them outweighs the financial cost, these tend to be retired people who spend months at a site that friends and family also use.
In your case, with precarious finances in the picture, I’m afraid it would be a terrible idea.
I come from a place where these sites are very popular and a good friend is the accountant for a couple of them. I’m quite familiar with them.