My inclination would be to use miles to get to places where the purchased price in a premium cabin would be prohibitive, and/or where it'd be unlikely you'd travel in the course of most years, due to the time and distance involved. Europe is available fairly easily on paid or upgraded itineraries, while places like Oz/NZ or especially Africa are not easy to access comfortably from North America.

I personally would look at the CX J award NA <> Africa myself - for 125K miles it's a heck of a deal, as it would likely involve CX's new J product for the transpac by the time you travel (which is probably the equivalent of most other carriers' F) and you'd get a twofer out of it - HKG (from which a stopover could be used for visits to China or other south Asian places) and South Africa. That's a lot of the world for 125K miles (I'd recommend J because CX's F service HKG-JNB is seasonal and you might end up with no F offered on that long sector.)

Maybe LHR T5 will be the salvation of BA, but it's going to take some doing to convince me that any optional travel through LHR is a good idea.

FWIW, though, I still think the LA J award to deep South America is the best bargain in the AS award universe.