Just be aware that CX YVR-JFK in F is not the same service as JFK-YVR. It is a trimmed-down service, not CX at its finest; so if looking for the F experience you have to fly JFK-YVR and not YVR-JFK (or pick a different route).

I hate to quibble about the purchased segments rules, but I've spent a bit of time with several airlines discussing this, so unless it has changed recently, the purchased segments have exactly the same restrictions as the "free" segments and cannot be used to circumvent the LHR routing restrictions. If you really want to go to ATH and DXB, then ATH has to be on IB or AY and not BA.

CX has a particular good F service HKG-NRT (because it has to be much better than JL on this route to attract any Japanese F business). Makes a very nice short flight (much better than YVR-JFK for example). The kaiseki meals have been cut back and are not so lavish as they used to be, but still well done (and probably have a catering cost of USD 100 which is quite a bit for a short flight).