If you own the title free of a charge then you could in theory transfer it to whoever you like by using the Land Registry's TR1 form and submitting it to them with their Form AP1 to register the change in ownership.
The difficulty is that during the sale process of the other title the Buyer's and their solicitors are likely to become aware of the discrepancy of the property marketed to them and the land shown on the one title over which the lender has a charge. The problem will be revealed and the lender will take steps to attempt to secure their right to sell the second title. If you executed a mortgage deed during the remortgage or original purchase for all of land (ie. including both titles) then you would have accepted that there is a charge over both titles, the fact that it has not been registered does not change this and a Court would likely take the view that the lender is entitled to have a charge registered over the second title (and to sell it).
If the buyers purchased only the first title repossessed by the lender and did not become aware of the second title, but there were debts outstanding on the mortgage following sale then the lender would look to claim this from you. The courts have the power to set aside transactions which were made with a view to defrauding creditors and that would be what transferring the second title to a relative would be I'm afraid.
The lender is under a duty to get a reasonable price for the property they are selling, if they are found not to have done that and you still owe them money then you could ask a court to re-assess the amount owed to the lender. If you suspect they are not doing this then you should speak to a property litigation specialist or write to them personally expressing your concerns. If you continue doing this it will corroborate any claim you have against them later.
I would advise you to be honest with the mortgage company, there is obviously a tension between on the one hand claiming that the lender is not getting a reasonable price and on the other being aware of the second title and not disclosing it.
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