The miles you earn are dependant on the carrier you are flying with and not the booking airline. Your ticket is a codeshare with BD, however the actual flight is operated by LH.

It seems that the booking class S on BD maps over to M class on LH.

Look up the BD earning table for Lufthansa operated flights, and M class earns x0.5 miles, hence you earn 300 miles.

bmi earnings chart:

http://www.flybmi.com/bmi/src/html/d...earnmiles.html