I'll echo a previous comment that you may be overthinking things too much.
From what you've told us, you have no planning permission on your land. You have around an acre of essentially unmanaged woodland, which we'll presume for now would be regarded officially as woodland rather than scrub or wasteland. You might officially get away with a structure related to the 'woodland' if it is regarded is such, but you may find yourself needing to justify it's existence to the authorities. A tiny area of woodland without a management plan in place is a bit borderline for requiring a structure.
My experience of being involved in several projects (woodland management, historic buildings techniques, permaculture, regenerative farming) has been that regardless of good intentions, doing the right thing by the planet, there will always be somebody in the locality who objects and makes life difficult, stirring things up with the council. Every time without fail! Given that you are living in a van and coming and going to a piece of land you have just bought, it is likely somebody is worried you are aiming to live there and will moan to the council who will also be worried you are trying to live there. Don't give them any excuse to hound you. Start small and simple, if it turns out everyone in the locality supports you, then get more creative.
And yes, there is a 28 day rule regards camping in a tent on your own land, but it doesn't apply to vans or buildings (which would have to comply with building regs, planning permission for residential use and council tax). Even staying legally in a tent is likely to get the alarm bells ringing with locals though, it would be sensible to avoid that for a year or two.
I really don't mean to put a downer on things. But probably best to keep things quick, simple and easily dismantleable if there is trouble, or make things really well hidden (including from drones these days). Paranoid? No, I've just experienced people's fear of change in a neighborhood.