Not sure about BMI's fleet but there is a glut of F100's on the market after US Air and American ditching their fleet, not to mention one of the Brazilian carriers with a few dozen grounded and on the market at the moment.
Cheap modern aircraft can be hard to come by but the F100 has become just that (mostly mid-90's aircraft), hence Germania, helvetic, EU Jet ops etc. are availing of the opportunity. Can't imagine leases on F100's would be much more than those of a 146 or an older EFIS 737, whereas operating costs would probably be better given the age and weight.
Is the F100 the 737-200 for a new generation of locos?