You should write to your tax office and sya something like:
I am advising you that our property at 123 Cosy Close, Little Snoring, Loamshire is our principal private residence however because of my service in HM Forces we are living in Service accommodation at RAF Lincolnshire/Ramstein/Wherever. The property is currently let.
This means that if you sell the property without having lived in it (although it was your intention to do so) you can demonstrate to the Revenue that it was your private residence. HMRC will write back to you confirming that it is your PPR. Let's say that your have two rental properties - declare the one that you intend to sell (or the more valuable of the two) the PPR. When that's sold, write to HMRC and advise them which is your new PPR.