Only offset against mortgage Interest. not the repayment part.I think you will find that serving overseas is still counted as being a uk resident and therefore tax is paid. I know when I was in Cyprus the BFPO address kept us taxable. if you play the numbers right you will pay hardly any tax anyway.
from the rent per month you can deduct the following:
Agents Fees
Mortgage
Any insurances on the property (buildings and life insurance for endowment if your lucky)
Contract fees including inventory (don't claim it twice from the RAF though......)
Repairs but not improvements but it is amazing how many things fail like radiators or bathroom suites
Fair wear and tear to fixtures and fittings
You only pay tax at standard rate on any profit. If you are near your higher rate bracket let the wife claim it!
Where are you going?