You can't claim mortgage interest against the rent on the former PPR since the loan will not have been raised in order to purchase the property. Once the mortgage has been cleared, any re-financing will not qualify.
In any case, the fact that the rent would only cover repayments is not relevant, as only the interest element of the payment would be deductible.
From the details posted, you would be better off seeking to increase the mortgage on your new PPR if possible, since the repayments on that will qualify for PPR mortgage interest relief. Or run the numbers again and decide whether it's worth retaining the PPR - you should calculate based on about ten months' rent per annum (to allow a month void and a month's rent on general maintenance / expenses other than tax liability).