Manchester has been roading cargo to BHX for years, maybe the 2nd A380 is part of the reason for the 3rd 777 at BHX. The A380 is planned at 5 tonnes of cargo per flight and the 777 up to 25 tonnes, depending on passenger load. Why does it depend on passenger load, because baggage loading policy can have a bearing on how many baggage bins are used thus reducing available cargo space.
To answer a previous comment, some cargo does have time constraints on it so if it doesn't travel there can be costs to the airline. This can result in a light pallet or bin traveling, as it is high priority, and a much heavier one waiting for the next flight even though the aircraft still has an underload.
Just glad I don't have to get involved with that juggling act anymore!