Because you're initially going to be letting the property it's only reasonable that the mortgage lenders consider this on a BTL basis. Yes, this will make the interest rate higher so I'd suggest looking at the shorter-term mortgage rates rather than committing to a particular "product" for 5 years. That way, if you return in (say) 3 years time you can move on to a cheaper homeowner rate much sooner.
If you were to keep your current house and the fixed/tracker rate on that is expiring it may be cheaper to stick with the lender's standard variable rate.
We are overseas and when we rented our house out we stayed with current mortgage and got Consent to Let. We are now looking to sell and then buy a bigger place. We will rent it out for between 1-5 years but it will be our sole property and we intend to live in it when we return. I'm being told that we have to get a BTL mortgage which is more expensive. HAs anybody done better? (I am paid in UK as British Crown Servant and we are only looking for 40% LTV ratio if any of that helps?!)