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Both TAP Portugal (through Star Alliance partners like United and ANA, indirectly) and Iberia could be used in this combination.
This is fine.
I generally find that two separate booking references work best anyway, even when flying the same airline in both directions.
By doing this, you can fly nonstop both ways.
Best advice is to use a free award search engine tool like PointsYeah.
Typically, booking with Iberia uses fewer Avios than booking with British Airways.
If you are flying economy, look for Blue Class on Iberia for additional savings.