I sent this message to BA via Twitter:

Hi I have a couple of questions about the status match I received to BA Gold. 1) What constitutes a qualifying return flight? Are JFK-LHR-ATL or JFK-LHR-JFK qualifying return flights? 2) Can a qualifying flight be booked via Amex Travel as long as these are BA-marketed flights with BA flight numbers?

Response:

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Hi. Eligible flights are defined as those flights marketed or operated by British Airways and those flights marketed and operated by Iberia.

- 'Marketed' means the flight will have a BA flight number (in your itinerary or on your ticket).
- 'Operated' means the aircraft that you travel on is a British Airways aircraft (including franchises and BACityflyer).
- 'Marketed and operated by Iberia' means the flight will have an IB flight number and the aircraft you travel on is an Iberia aircraft (including franchises).

Reward bookings and agency or industry discounted bookings are not eligible, but you can book via Amex Travel as long as your flights are eligible and not discounted.
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So, it looks like Amex Travel or other OTAs are fine. She didn’t really answer the question about the routing though.