All our BTL mortgages are repayment including a recent one from The Mortgage Works, a subsidary of Nationwide.
It is reasonably short term (20 years) as we are essentially using it as a pension vehicle. When the fix comes to an end, we will try to pay off a lump sum and increase repayments to reduce the term further. When broken down we are effectively saving a significant amount each year, mainly covered by our rental income/tax allowances.
This probably is not the most tax efficient way of doing things, but I like arranging my finance in a way I understand. Interalia we might want to use the properties for something else, indeed we see one property as a potential second home at a point when we have more time, and a chance now to get to know the area.
Plus my father dabbled in rented property as a retirement hobby, but failed to recognise when he was struggling to cope, leading to a cycle of run down property and poor tenants. The estate agent says this is not unusual and that in this area blocks around 25 years old often have a number of elderly owners. The paperwork, eg Powers of Attorney and probate is complicated, and we have had to essentially lend my mother the money to carry out a comprehensive refurbishment, plus overseeing the works has been time consuming and stressful. Luckily he did not have mortgages. My guess is that at probate the lenders would not have been willing to reassign mortgages to my elderly mother, so there would have had to have been a firesale of poorly maintained property, which using a probate lawyer would have been inefficient and costly. As it is, though after a lot of hard work on my part, she has assets which provide sufficient income to pay for her care provision, and which should sell promptly. A bit of an essay but our intention is to ensure that we don't land our children with a similar burden.
BTL mortgages can be quite complicated as there is quite an array different fees and terms. (Some don't like novice landlords, rates improve dramatically if you have a 40%+ deposit, etc) When we were looking a significant number were only available through brokers and I was finding comparing the alternatives difficult. Things may have changed a bit. A quick glance suggests you can now buy TMW products direct. However we had a very good experience with our fee free broker (London and Country) who listened to what we wanted, suggested some alternatives and gave us an agreement in principle within a day or so. All the paperwork was routed through them, and it was helpful to have someone to ask questions of who I felt was on my side. (Not naivety! Just he wanted the mortgage to go through like we did.) Happy to PM the name of the guy we used, simply because he deserves something for his efficiency.
Good luck and I think you are on the right track.