I wouldn't recommend handing the keys in. Can you not move back into the empty property and let the rented property go to reduce your outgoings a bit?(posted 8252 days ago)Have you spoken to your lenders for your other outgoings? If you tell them that you're struggling, they should be willing to accept a reduced payment for a while until you get yourself sorted.
If it does come to it, get two surveys done on the mortgaged property as proof of the value. This website should give you a good insight to how Lenders deal with repossessed properties and how they are known to sell them for less than they're actually worth.