I don't know how it works with Iberia's FFP, but normally BA is not exactly generous with miles. I am a Qantas FF and whenever I fly BA in discount economy (all booking classes except Y & few others) I get only 25% of the miles flown. I guess it may be the same for IB.

Instead, I get 100% of miles in most classes for flights on IB, so if the mileage agreement is reciprocated, fly Qantas from LHR or FRA to SIN and you'll get all miles credited. Of course in business or first, you won't have such problems.

Hope I could help you.