IIRC, any fare booked on the BMI website called a "Tiny Fare" is booked, "ticketed" and processed as if it were a booking with bmiBaby. Indeed, in Aberdeen Airport there are bmiBaby signs throughout the terminal BUT no bmiBaby aircraft has, to my knowledge, ever landed here.
Seems the two "companies" are becoming more and more streamlined, what with the continued change of bmiMainline in to a LoCo carrier.