Hi, first ever post, but hopefully can help a bit.
I am self employed and was sick of pouring more and more money into pensions that were worth less and less so persuaded my wife that when we moved we should keep our flat as a rental property.
Basically this is how it worked (for us), bear in mind we did it 3 years ago when the banks were giving money away like it was going out of fashion!
We bought our flat for about 60k, when we came to move it was worth about 120k so we had plenty of equity. So we remortgaged with a buy to let from Birmingham Midshires for 90k. We paid off the existing mortgage with the 60k and used the 30 k as a deposit on our new house which cost about 200k.
The most important thing with buy to lets is the yield, basically the rent vs mortgage payments(interest only). If the yield is not good enough then you won't get the mortgage - we would struggle to get a new deal now unless i pay off some of the money we owe on the mortgage.
I am glad we did it although the first year was a struggle - we had 3 months with no tennant, as well as the fact that our mortgage on our new house was huge compared to our old one. We use a letting agent who takes a bit but removes all the hassle.
Best idea is to speak to a local letting agent and see how much you would get per month then speak to someone about a mortgage.
I don't regret it, hope this helps a bit.