Chances are that the ISI owns half of the beneficial interest in the house, rather than the house itself. The house probably remains in the names of the original borrowers who can sell it, or let it be repossessed. Or if they are real cheeky, go and live in it.
The lender should apply to the court for a possession order. With a possession order the lender can then sell the house.
Alternatively, if the original loan agreement permits, and the house was a "buy to let", the lender could appoint a Receiver who could sell the house on it's behalf.