Your'e options are limited, but you do need to address your financial position.
I assume that the investment property is in negative equity & as you say, you can currently neither sell nor let it. Loan amount nor mortgage not stated on your post. I assume that you are currently meeting the full repayments.
You also have a home loan. I assume that you are currently meeting the full mortgage payments on this loan.
Income & expenditure details not advised. I again assume that you have made all of the available cutbacks in expenditure and you are still unable to meet all of the household bills.
Based on this summation, you clearly can't continue to pay both mortgages.
Before meeting the Bank, complete a brief household budget. Include all income/expenditure before loan repayments. Make sure you address all household expenditure, including irregular payments such as car tax, insurance etc. Divide annual payments by 12 to give approximate monthly outlay. Also include some small element of social expenditure. I.e. Make sure that you give yourself enough to meet all of your regular outlay.
The balance is what is available to make loan repayments. Deal with the Bank on this basis and state that you will continue to fully co-operate in selling/leasing the investment property. Don't be bullied into paying more than you can afford. You are entitled to use the MARP agreement in terms of repayments on your HL. The bottom line is that you can only pay so much, and provided that you are open and honest with the Bank, there is little that they can do to force you to pay more.