Hi Sara, I am a Hong Kong local. You can surely do all the life essentail in HK in Englsih here. I also know many expats here who can only sepak Englsih where they are living in "Expart area" like Central, Island South, Sia Kung or Ma On Shan.
It depends whether or not you want to explore local culture or be closer with your local colleague, learning Cantonese(instead of Mandarin) is really useful.
HK local can understnad Mandarin but seldom say it, 99% of the time pepole chat in Cantonese in work place unless there is someting important that you need to know, you might feel isolated or else you need to pay etxtra effort to fit in the group. In local restaurants and shops some waiters/shop keepers only have very limited English level(i.e. price, greetings).
If you want to stay >2 years, I suggest you to take some Cantonese course after you come, than you can truly understand HK culture instead of confinding yourself in the expat group.
Welcome to Hong Kong!