Most mortgage providers won’t lend to BTL borrowers unless they already own their own home, and some will stipulate that you have to have been a homeowner for over a certain amount of time.
Generally speaking, BTL mortgage providers favour customers with landlord experience, as evidence of a strong track record of managing rental properties will provide additional support to your application.
There are however some providers who are willing to consider BTL applicants that are first-time landlords or even first-time buyers, although you may be limited in your choice of lenders, and there could be caveats attached.