Don't be worried. As I said earlier, you have plenty of time. I wish someone who can work the search function could find the right thread (it has loads of advice and information).

In the unlikely event you miss your ABZ flight, BMI may allow you to travel on the BA flight rather than wait around for the next BMI one, but it's entirely discretionary and they don't do it as a matter of policy.

To expedite your transfer? Not much that will help but if you are running very late into LHR, there is a BMI transfer desk at the Flight Connection Centre (just after security re-clearance) where you can get help.