If AA in ZRH don't want to issue this ticket, I think Switzerland's membership of the ECAA could prove critical. Wikipedia claims that many non-EU states (including Norway & Iceland) have, in fact, signed the ECAA agreement, but that Switzerland isn't (yet) one of them.

Personally, if AA in Switzerland wouldn't play ball on this, I'd get the itinerary set up by the RTW desk for ticketing by AA in London. You'd be leaving from London anyway, if your physical presence were required for any reason. If the ticket were all-electronic, AA in London could probably remotely charge your (Swiss) credit card with the correct GBP amount. You would probably lose out on the exchange rate conversion, which is significant but not show-stopping.