The BD policy is listed on their website for the travel trade at:
http://www.flybmi.com/trade/en-gb/tr...ge-advice.aspx
Interline allowance
If a passenger is travelling on a bmi shorthaul interline fare and then onto a different carrier on the same ticket, bmi will honour the luggage allowance of the onward carrier.
I always carry a printout of this when connecting, shuts most check-in staff up.