The law states that if freehold land becomes ownerless in the UK, it can pass into the ownership of the monarch in a process called "escheat".

BBC: Beds, Herts & Bucks

If for any reason he cannot locate the customer after reasonable and documented efforts as agreed to in writing with FINRA staff, Prim shall forward any undistributed restitution and interest to the appropriate escheat, unclaimed property, or abandoned property fund for the state in which the customer is last known to have resided.

FORBES: After Borrowing From Customer, Stockbroker Ordered To Pay Restitution