It is a matter of choice. I would prefer to stay at Kingston Lacy CL than on a Haven site but I am not knocking other peoples choices. We all have different priorities and make our own decisions. Kingston Lacy is expensive, now £32 per night, but if it is over priced people will stop going there. The site attracts me because it is in the grounds of a NT stately home. The site facilities are limited but within a few minutes walk there are NT facilities including a cafe. We are going there later in the month and if we are the only on the site I will conclude that they have pushed the price too far.
I find comparisons with continental prices unhelpful. They often have very different traditions, different population densities, different planning rules and different land values. I suspect that if comparisons were made with other densely populated countries like the Netherlands the price differences might be much narrower.