My girlfriend missed her BHX-EDI last summer because a train broke down between New Street and International. Two-hour delay -- she arrived at the airport as the flight was taking off.
The guy said that she'd have to buy a new ticket (£180 or something), but then took pity on her and charged her £60 instead. It wasn't a change fee but definitely for a new ticket -- but given that BA doesn't sell cheap domestic oneways, I'm not sure how he pulled it off. I did have the receipt but I think I've lost it.