I don't know, my guess would be
If you didn't pay the mortgage it would be repossessed and sold at auction probably for a lot less that if you put it on the market. If the auction sale didn't cover your debt, then you would still own the building society �x,000s of pounds which you would have to pay back. You could to try and add this debt to the mortgage on the property you live in.
If you have lost your rental earnings from the rented property, your building society might judge you will not be able to pay them back on the living property.
Do not let it get that for. Keep up the payments on the rented property and sell it yourself asap.