The same used to happen on bmi. If you were in business class on a flight where they operated buy on board in economy (e.g. domestics or short European flights) they'd not be able to get you anything complimentary from standard if business class had no food (was common if people were using their paper upgrade vouchers). Although when they did the free meal deal for status holders they'd happily swipe your diamond club card and give you a meal, but if you didn't have status then I think paying was your only option. I don't know if they changed their policy when they removed the diamond club meal deal.