If trying to book a revenue ticket, you are right, the BA website does (or in its version last year) struggle - even worse if you throw in an Open jaw LHR-CUZ//AQP-LHR, as it doesn't want to know about Arequipa.

Best bet - have a look on the Iberia website, then call up BA and get them to book it under the relevant BA/LA(LP) flight numbers.

Finally - avoid Iberia long haul flights at all costs.