Just to update from my original question...
Halifax (my bank) offered me a mortgage, so I put in an offer on a house, then Halifax changed their mind. They'd made me a "mortgage promise" based on 75% TLV, then a week later decided their rules had changed and they now only offered 70%.
So, with an offer in on a house, I went to Santander. Santander told me I would be charged extra for a BTL mortgage. But, if I wanted a homeowner mortgage, but became unable to live in the house through RAF moving me around, then I could get permission to sub-let for no charge.
So I told Santander I wanted to live in the house, and would do as soon as I could move nearer. They gave me a homeowner rate, on 75% LTV, with permission to sub-let.
I picked a house just down the road from where my parents lived. This meant they can run the rental for me (and I pay them the letting agent fee) as I trust them far more than a random company who don't actually give a toss about me, but who might be expected to make important decisions on spending while I'm OOA.
I chose a house I'd be happy to live in, in a part of the country I'd be happy to live in. If I leave the RAF and need the money for a house then I can always sell up. But if I bought a place that was purely a rental property I'd have to sell it should I ever need somewhere to live.
The downside of picking a place you're happy to live in, is that it may reduce the rental Yield you can expect. Often the properties that have the best Rental Yield aren't always the ones you'd want to own to live in.
For anyone looking to get a BTL while they stay in SSA, SFA or wherever - I'd say go for it. Having someone else help pay your mortgage while you're enjoying the cheap forces accomodation is a good thing.
Possibly the single biggest, most important and most expensive (if you chose badly) decision you'll make is which tenants. You can't spend too much time checking them out, either with credit checks, interviews, chasing up genuine references from previous landlords etc etc. So far I've got lucky, I've got a working single mum with two kids, who's very house proud and who really thinks of the place as a "home" rather than just somewhere to stay.