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I am afraid it is not as easy as just claiming the land. Just because the Land is not registered does not mean no one owns it- about 20% of UK land is unregistered and it just means that the ownership of the land in question has been in the same person's ownership for so long that it has never been a requirement to get it registered.
In order to claim land, you have to have possessed the land exclusively without objection or the consent of the legal owner for 12 years. Once you have possessed for 12 years, you can then make an application to the Land Registry for possessory title.
I hope this sets out the legal position.