Simple. If you fly within Europe but your trip involves a change of plane, you have to pay an extra 7,500 miles for the redemption (round trip, and of course less for cash and miles).

This is presumably to ensure that the airline you redeem on (usually Lufthansa) is fairly rewarded for the cost of giving you two seats on two flights, with two meals etc.

Given that BMI's European network is not great, this policy is discouraging for anyone wanting to fly anywhere in Europe except for Germany, Switzerland, Austria or Portugal, when a plane change will almost certainly be needed. However, you can't argue with the reasoning.