matthandy, I checked fare rules for a selection of fares from SYD to LHR, and it seems like they are rather restrictive with regard to combinability).
I think EU fares are less restrictive.

Henkybaby, I checked the two cheapest WTP/CW fares from AMS to JFK, and they are combinable with any BA fare (restrictions only when combining with OW partners' fares). Let me know where you're trying to go specifically, and I'll check.

I was told by the local office that all BA fares are combinale. THis must be valid only to fares originating in European countries...