You can return the keys to the mortgage company normally, voluntary repossession, however the mortgage company will then force a sale (which will take ages) and you'll owe the shortfall between the mortgage and the cost of sale.
I believe you'd be better off declaring bankruptcy or IVA, moving abroad and waiting for it to fall off your credit file. If you speak to CCCS or someone free like that, they'll give you advice on the best order of declaring bankruptcy and handing back keys.
Legally, debt is not international so if you did do a runner so to speak, they have pretty much no power to do anything to you (although they may say otherwise, and hound your family etc), the thing about life is, you never know what will happen, so I wouldn't want to close a door, and have all the stress and worry of leaving all that craziness behind.
If you can sell it for more than the mortgage shortfall, just do that! It would be insane to think of doing anything else.