DXB-HKG, for the FCL in DXB, as well as A380 to HKG rather than the (perfectly adequate) 77W from GLA.

Is it worth it? Depends on what you value a Skywards mile at, if you're looking at it from a cash point of view and what else you were going to use the miles for. A J-F upgrade on DXB-HKG gives decent value - and you should get the car in HKG which you won't do in J.

Personally, I think it's worth it, if only for the shower, onboard bar and FCL at DXB.