HKCB, my understanding is that in the public system you fall into a catchment area depending on where you live. You may be asked to furnish proof of residence. Some public hospitals may be less strict about this than others. I have heard Queen Mary is the strictest - and this is the one that most expats want to deliver in as it might be the most English-speaking and expat-friendly. If you really want to go to one hospital over another, you could try getting a reference from a private doctor that states that you have a condition that requires you to be in a teaching hospital.
I delivered in United Christian Hospital in Kwun Tong. The check-ups were in Hang Hau. You might just fall into that catchment area as well and could consider it from the point of view of convenience of doing the check-ups. I just realised that I could have decided to deliver at Queen Elizabeth as well. I think Queen Elizabeth and Prince of Wales might be better hospitals than United Christian, being bigger and teaching hospitals with more experienced staff.
Regarding English-speaking, that is not a major concern. You may find a problem speaking English over the phone but in person the nursing staff can all speak English, even in the Hang Hau clinic where I was often the only foreigner among like 100 women. And of course, the doctors are fluent in English too. My delivery was pretty much handled by midwives who were great, with their English and their coaching skills. The doctor who came in to see me about three times was a complete asshole but luckily I saw her only once during the delivery. You can read about my experience by searching United Christian Hospital on this forum. My main grouse would be that I could not secure an epidural...I think it's very much up to the whims and fancies of the doc on call.
In terms of antenatal care, I found the public system very good and the approach of the doctors pragmatic. In fact, I stopped going to a private doctor in the third trimester.