Technically the Queen (the crown) owns all the land. In practice it doesn't make much difference, but it does kick in if a freehold suddenly has no owner.
In the US if an owner of property can't be identified, then it's given to the state to deal with and in France it goes to the local authority.
In most of the UK, the land is given to the Crown Estate (not the monarchs personal property but to fund the country). The exception is Cornwall and Lancashire, where it's given to the relevant duchies (i.e. the prince of wales and the monarch), who by convention sell it and donate the proceeds to charity.