Is the property sound , clean , tidy and all up together? Do you pay the going rate ? Or is it cheap as there is work that needs doing?
If you pay a fair rental , and there are no immediate expenses due (boiler / kitchen / bathroom refit) then good chance new BTL landlord will keep you on as existing tennant .
Toys19 has a few properties and could confirm , but I am pretty certain its easier to get BTL finance when the property is already let at a sensible rate . Alot depends on purchace price and gearing/deposit.
You might get a rental increase, or be flexible over works carried out by new LL .
All is not lost , but keep an eye out for somewhere else in case the flat goes to market and a FTB buys it and gives you a months notice.