A few comments -

First, you should have a look at this thread regarding your proposed flight to the Canaries. GB Airways, who fly this route for BA, have been sold to Easyjet. You may or may not be able to include this route in your RTW.

If you intend to use the AA mileage program, you won't get any mileage credit for the BA SEA-LHR flight, You would get credit, however, on BA's YVR-LHR flight. That said, if you're relying on AA to issue the ticket (and you should in order to save money on fuel surcharges) you probably ought to consider giving AA one of the over-water longhaul segments; SEA-DFW-LGW maybe, or use your available N. America transcon and take CX's YVR-JFK flight and connect to AA in NYC to London or elsewhere in Europe.

You can phone the AA RTW desk and have them put the PNR together then send it off to their GSA in South Africa (in Cape Town actually) for ticketing and pricing.