AHH! No one has mentioned the most important thing -- heaters take SPACE! I've only spent one winter (last) in HK, so I'm not the expert, but we were going to buy one last year as we were settling in. Then we never really got cold enough at all to need one. In the end we never bought one, and for the past 9 months we have been VERY grateful to not have one taking up non-existent space in our closet!
So if you're just moving here, then I highly recommend you first come, get your stuff here, get settled (because all your stuff if going to be a bit of a nightmare to unpack and organize when you go from a UK-sized place to a HK-sized place), wait for winter to come and then only get a heater when you really come to a point that you need one!
I am REALLY interested to see how the "bad" insulation and "single glazed windows" that you have here somehow drastically create a moldy, miserable winter that you don't (really??) get in the UK. I find the commends in this thread baffling! Our last apartment in the UK was AWFUL - only electric heaters, single glazed windows that constantly had a draft coming through them, paper thin walls that got completely moldy and damp from exposure to the outside (interior building walls were ok). It was 11C in the bedroom most nights!! HK is a dream so far - at least the single glazed windows here were made in the last century and close properly!