BA GGL and BAGGLfL are treated by AA as BAVIPs but I would not assume you will always be met - MIA with all the Int flights to South America is CK heavy who will get priority over BA GGL - I have been told that by AA MIA premium services.
You may have bad weather (summer thunderstorms) and also as it’s a flight to UK there could well be a document check depending where you are coming from. Also I have done a few MIA-LHR’s on AA and they have all been on the low D number gates- one end of the terminal - if your flight arrives at High D’s or E gates you will be pushed for time.

As you are BAGGL for life (and BA canx your flight in F) - can you ask BA to meet you off your inbound flight to MIA?