For those who have not memorized the AA schedule, this is the LHR-MIA service.
What matters is the allowance printed on your e-ticket receipt, not an itinerary or other document.
US law provides that for tickets originating in or destined to the US, the baggage allowance of the marketing -- not operating -- carrier of the first segment, applies to all segments of the same ticket. While it is more than probable that BA is the marketing carrier and thus the BA allowance applies, it is worth asking your TA as it is always possible that it ticketed otherwise, in which case the AA allowance (or non-allowance) as the case may be applied.
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