1) Do you mean you have let it under the rent a room scheme or let it totally, i.e. you do not live there at all ? If you let it totally, so it is not now your PPR then provided the letting is registered with the PRTB then mortgage interest is allowable ( at 100% up to 2008 but restricted now to 75% for 2009 )
2) If you have a loss on another property then that can be set against profits on other rental properties in the same year or carried forward against rental profits in a subsequent year. It can not be set against other income.