BA could greatly improve the situation by changing their rule so that the same baggage allowance applies on any set of flights covered by the same fare.
That would prevent people from receiving a higher baggage allowance in all the more manipulative cases: half round trip fares to increase baggage allowance on the lower class half of the trip, end-on-end combinations with short higher class journeys, AUP one segment to get cheap bags on outbound and return, etc. It would still allow people with a simple stopover to a higher class flight to receive the reasonable and expected features of the fare they paid.