I'm unsurprised at the response to be honest and it's much as I expected.
I've had a word with the dealer in question and they have stated their case for charging as follows:
1. BMW UK/AG do not cover the cost of loan cars even for warranty work.
2. The loan cars depreciate with mileage (obviously)
3. When being sold they have to be prepped to AUC standards
4. When a loan car is sold another car has to be bought to replace it
5. Drivers of loan cars have to be insured
6. Margins are so tight that if loan cars were not charged they would not make any money service/warranty work.