Wales might be an option considering their One Planet Development rules - http://www.oneplanetcouncil.org.uk/ - or for more detail http://www.oneplanetcouncil.org.uk/wp-content/uploads/2014/0...

While planning rules make it difficult to make new-builds in the countryside, you might look at fixer-uppers in rural Ireland. If you're a UK citizen you still have the right to live in Ireland.

In many cases councils are eager to have people fixing up vacant houses, and grants are available. This is how I came to have thatched cottage on 3.5 acres in Co. Offaly. And I had ~40,000 euro, not 1 million pounds! I made a site to find properties like this, go to gaffologist.com/ and choose "advanced" if you want to select for acreage (warning - it's buggy!)

https://www.youtube.com/@MossyBottom is worth watching on this topic as well; he lives off-grid in rural Ireland and is fixing up an old cottage.