See under "Allowances on mixed cabin journeys" here:
https://www.britishairways.com/en-gb...age-allowances
Originally Posted by
windowontheAside
However, fairly recently on here I was educated by some convincing arguments that the baggage allowance only applies to each connected set of flights, rather than the full itinerary so a typical round trip journey would be considered as two separate trips and the baggage allowance for each one independently would apply.
BA has a policy as per above that covers this, other airlines may not and so if booked in different classes the outbound and return allowance may differ.