In HK you can live there speaking English or Cantonese only, or only Hakka which is mostly in the NT areas, though those damned punti laos have occupied most of the traditional Hakka lands too. It is perfectly possible to live in HK speaking and reading English only. Road signs, maps announcements on PA systems are in Mandarin, Chinese and English. There are English news papers and even TVB pearl an English TV channel. Whenever I traverse the mid levels you seldom hear any Chinese spoken at all since it is stuffed full of expats and their irritating useless trophy wifes and or husbands. It's called a white ghetto for a reason.
However in speaking only English (I presume) you will severely limit yourself on the jobs front as the younger locals universally will speak English. It is heavily accented though but perfectly understandable.
For example here
Its kinda funny because my time spent overseas means I wonder if I sound like that to Chinese people (I don't people think I am native, something to do with being raised on TVB I suppose) Though this plays haywire with my UK accent as simply it is a mishmash or different UK accents languages and tones. Thus everybody recognises me instantly on the phone.
It is highly useful to pretend NOT to be able to speak Chinese though as people will say nasty things behind your back thinking you won't understand. Heh an anecdote is my aunt runs a shop I had to help out once and she said something nasty about a black man who came in. You know Si huuk Gwai, unfortunately for her this man had lived in HK for many years and understood her and was somewhat offended.
If the company you work for deals with China (most of them do) you'll need a good grasp of Mandarin too. And thus without sufficient language skills you may well suffer on the jobs front since the locals will work harder (due to being indoctrinated), faster, and are highly educated and motivated due to face and shame issues. Probably why my lazy ass is in the UK where I can work 30-40 hours a week, while a similar job in Asia would pay about the same (but pay less tax) working 70-110 hours a week. Companies will want value from their employees and thus will want more skills. So they see a CV with English only then you will be placed lower than somebody with multiple language skills.
Except for English teachers of course.... it is actually preferred that you are ONLY an English speaker so as to be more authentic. Or just to look the part (TCG has seen Russians teaching English in HK due to appearance sake. I had to speak Russian to the poor girl to communicate!)
Though this is dying out a bit. My friend J in HK and has been there for a decade has started to see the trend reverse. Perhaps it is because there are far too many short term travelling teachers who teach badly.
Which can sometimes result in some uber hilarity as displayed here