Thanks for the info, raph.I tried some dummy bookings, expecting to find AA offering higher miles prices but no YQ. In the event, I found AB awards could be booked online with ba.com but not aa.com who said they could only be booked by phone. (And the Dublin office is now closed. )

Comparison for a 2-day trip LGW-NUE in December (NUE carefully chosen as BA don't fly there).

- BA: 9,000 BA Miles + £102.20 direct (from LHR with BA connecting with AB in DUS = higher surcharges)
- AA: 10,000 AA Miles + presumably lower surcharges
- AB: 15,000 AB Miles (? assumption) + high-ish surcharges

I then looked at revenue bookings:

- BA revenue booking: £137 + £73.20 surcharges = £210.20
- AB revenue booking: €80.98 + €95 surcharges = €175.98

I compared just the one trip on specific dates, so it would be dangerous to draw too many conclusions. However,
- it is worth comparing different redemption possibilities;
- I'd have to be desperate for TPs or BA Miles to even contemplate booking AB flights at ba.com (the miles/TP calculator shows 0 for LGW-NUE on AB)