"Call costs �1.50 / min from BT landline & lasts approximately 5 min. Please get permission from whoever pays the bill. Over 16's only"
Says it all. As far as I know what they say is ture in that there is land in the UK which is unclaimed, but getting hold of it and more importantly doing anything with it are another thing.
From what I remember you need to do a hell of a lot of work tracking down the last person to own the land, this invloves plenty of solicitors who as we all know don't come cheap.
The key in this is that if it was so easy these boys would be out claiming it all for themselves rather than charging you �7 a call to tell you what you could learn in the library in about an hour.