In the U.S. we have big box stores like Home Depot and Lowes, and I bet a lot of work is done in their parking lots, though in my opinion it's pretty close to broke down on the side of the road.
You'll need access to power, unless you already have a solar array and all your tools are battery operated. You'll need some place secure so if you leave it for a few hours you don't come back to a break-in. I'm guessing this is more or less obvious.
Ideally, you have a friend with space in a rural town!
If I were looking for space to build I would research Tiny House communities (e.g. https://www.tinyhousecommunitybristol.org/seamills
), or Maker Spaces (e.g. UK Makerspace ď make things, do stuff and knit the community together"
). Or find like-minded folks and create your own cooperative?
As the skoolie market matures more opportunities crop up. England does not appear to be the epicenter of skoolie life-yet.
Heck, why not buy a buy here, find one of the several build spaces folks in this forum have offered, get the thing built and ship it back?