Am I obligated to return the ten pounds, and to whom should I return it, to my friend or to the lender, knowing that I cannot communicate with the lender, and I do not trust my friend?

If the owner of the money lent it to your friend, who in turn lent it to you, then you are obliged to repay what you borrowed to your friend, because he is entrusted with it by the owner of the money. You can ask him for a way to contact the owner of the money to return his money to him.

However, if your friend took the debt on his own credit, with each of you responsible for repaying his share directly to the owner of the money, then you must repay it to the lender, especially if he informed you that he paid it to its owner on your behalf.

If you are unable to find the owner of the debt after searching and despairing, then you are obligated to give this money as charity on his behalf to clear your conscience. If the owner of the money or his heir appears later, give him the choice between approving the charity or repaying the loan. If he chooses repayment, you guarantee him his money, and your charity will be a reward for you.