It's a nice enough idea but when you draw up an actual business plan, to make it viable and to earn any real money you'll have a hell of a start up cost and will need to do it on a vast (Eurocamp) scale.
Very quick figures in my head...
One tent rented out every single day from April to October = roughly 210 days, but realistically that's not going to happen. I reckon if you get it rented out 100 days you'll be doing well (your client base is very limited - you need to find people that want to camp, in your specific area, and that don't already have their own gear). Say you charge £50 a night (and I think that's expensive, bearing in mind it's camping!! and you're competing with budget hotels and youth hostels), that's £5k you've made.
Minus campsite fee, if you do a deal with a campsite and they charge you £10 a night for a basic pitch with no EHU (and if any site is as excellent as you say, they'll be fully booked in school hols, etc, so won't be quick to do you any favours when then can rent a pitch out themselves for full price) now you're down at £4k.
Take out cost of the tent - a decent tent that people are actually going to be willing to rent will set you back minimum £500. Groundsheet, carpet, SIM's, camp beds, sleeping bags/blankets, cooker/BBQ, table and chairs, plates, cups, cutlery, electrical equipment and the million other bits and pieces you'll need - and it can't be cheap crap, it's got to all be of a standard good enough to rent out, I reckon that's at least another £500. Now you're at £3k profit.
Wear and tear on your tent and equipment - because people won't look after it as well as if it was their own so I'd plan on £500 per year for replacements/repairs. £2.5k profit.
Insurance. and PAT testing of electrical equipment - havent a clue how much this will be, lets say £100 but I'm sure it would be more, now you're at £2.4k profit.
Your time, effort, and not to mention fuel setting all this stuff up, cleaning it and taking it down between bookings. Take into account cancellations because of bad weather, taking the tent down in the rain and carting it home to dry out then storing it before getting it out to set it up again 2 or 3 days later (unless you're going to pay the campsite to keep it pitched even though it's not rented out).
Seems like a lot of work for very little return unless you're doing it on a large (UK wide) scale.