Unregistered Land in the United Kingdom

I don't think you're going to get anywhere with this,

The reason is that, land registry deals only with registered land, therefore your request about unregistered land will be outside of their scope.

Asking them to confirm or deny that land can be claimed and lived upon is also a stretch too far. Since they are working only within their remit.

I know what you want, you are still looking for authority.
Let me answer your question, Yes if you can find a piece of land, no-one is using, you can live on it. Not necessarily free of hassle from other humans who may claim that they have more right than you, including (but not limited too) the monarchy and 'government'. Which you may have to contest through finding some-one who may purport to be able to 'judge' the situation fairly. However understand this, You are equal to them. You do not need permission from a human to live on any piece of unused land. You have the permission from your maker and in fact, if you look within yourself you have the duty to your maker to protect and live well the life he/she/it/ has given you.

If you want to build a house, keep it out of 'the system' by never claiming to 'own the land' since this notion is ridiculous, no human can 'own land' and don't expect that your house will ever have any financial value within the system you are now free of. Build it out of straw and mud and it if comes crashing down, build it again.

I'll probably be moderated, understandably

Phil, ALL information and rights like this should be used in school systems to educate the people instead of making them dumb to the points of law, the system makes it clear that ignorance is NO defence (so why don't they teach our children they they really need to know.

No moderation rock's ;)