CX F in a heartbeat it isn't even close, albeit the A380 BA F seat is pretty decent, the rest of the service with CX will be MUCH better as will the IFE, food, drinks etc. etc. If CX still fly the 744 to LHR you want to pick a 777 the seat and screen are better on the 777 and the F cabin is smaller.

You can elect to use the BA Flounge at LHR if you like. I prefer the CX lounge on the whole though for natural daylight and runway views.

I don't think there is any security prioritisation at HKG but it runs pretty smoothly anyway. You used to be able to hit a CCR like entrance to the lounge before the lounge refurb, I am not sure if this is still possible.

The LHR flights tend to leave from gates close to the wing but I don't think that CX give buggy service for more remote gates. The buggy costs little anyway even if you pay yourself.