I think the crucial difference that you fail to make is that unregistered land does not mean unowned land. Since the Land Registration Act 2002, land registration has become compulsory (I think, if my admittedly stale property knowledge is wrong, PLEASE correct me). However, until land is sold it does not appear on the register. There is no land in the UK that is unclaimed. A nice idea though, even if it slightly misconceived. :-)
If the property is not registered all you can do is ask around. In crude figures about 85% of the properties in the UK are now registered but only something less than 40% of the land area. There is a way of transferring unregistered properties between parties so that they never become registered, and such owners frequently do not wish it to be openly known who they are.
I don't think this is the case, the property has been empty for at least a couple of years and is in a bad state of repair, it is boarded up and needs to be done up and have someone living in it. - its a waste otherwise.