Some advice:
Start hunting early - seats are available almost a year in advance, and finding seats on short notice is VERY hard to do, especially when it's for three people.
Consider different routings -- BCN first, LHR first, that sort of thing -- because that might help your search.
Look at booking through the US, to save fuel surcharges. On many destinations, the surcharge is so high that just buying regular tickets becomes more attractive again. United charges no fuel surcharges on reward tickets, presently, so they're a good one to watch for.
Sign up for the ANA frequent flyer program, because their website search engine shows more options than the Aeroplan website. It's tricky to learn so be patient, but it's worth it for complex itineraries.
You only pay the booking fee when you book, so calling Aeroplan to ask a question or ask them to check availability is free. However, it takes a long time and frustrates the agent, so any work you can do on your own will make you and them happier and nicer.
Good luck!