It takes a more than two mins to get from T7 to T8 even with the airtrain. There are some longish walks to / from the air train stations to/from baggage hall / check-in desks but at least it's on the flat (mainly) and there are lifts and escalators to help smooth the passage

If I have read my walking app correctly it took between 10 and 15 minutes from leaving baggage hall at T7 (domestic as I arrived on BA1) to getting to the AA check-in desks.

Personally I'd take the bags to T8 and check them in there.