Thanks for the clarification. I had a feeling there was more to this. So I think the way to express this is:

• So longhaul to shorthaul, book 2 tickets over MCT (1 hour at T5 and T3, 90 minutes if a T3 / T5 change is needed) and you can expect to be rebooked, so long as it's 125 ticketing and BA metal.
  
• Shorthaul to longhaul, same circumstances, at your own risk. You can ask for assistance, if you were not at fault and can prove that you may get some leeway, but it's risky. If your second ticket hasn't started there is some "open to discussion" protection in the Conditions of Carriage.
  
• Shorthaul to shorthaul, same circumstances, at your own risk, but there may well be mercy shown or a £60 change fee. Note many non HBO tickets have on the day flexibility (and can now be rebooked online too, after midnight local time), free of charge. To be confirmed.
  
• Longhaul to longhaul, same circumstances, at your own risk. You can ask for assistance, if you were not at fault and can prove that you may get some leeway, but it's risky. If your second ticket hasn't started there is some "open to discussion" protection in the Conditions of Carriage.
  
• Not 125 and/or not BA metal, at your own risk.