Just wanted to provide an update on this - HKBN have let me off the remaining 18 months of the contract after providing a job certification. This is the process I went through:
1. Called HKBN up advising that I would be leaving Hong Kong and wanting to terminate contract,
2. They advised I could pay the liquidated damages (3k+), transfer the account to another, or provide proof that I was leaving Hong Kong to take up employment elsewhere
3. I advised them that I would proceed with option 3. I contacted my future employer and asked them for a simple employment letter stating my start date, location and HKID number.
4. Once letter received from future employer, I emailed it to HKBN.
5. They responded about 5 hours later to say all fine, charges will be waived and some further detail about the final bill and where to return the equipment.
I'm very impressed by their customer service - very responsive, reasonable and clear on what needed to be done.