I am creating an account just because I see your post. :)
A qualified HK doctor will be required to take a qualifying exam in order to come to the UK to work. That's why qualified HK doctors seldom come to the UK, on top of lower salary. I am not familiar with NHS trainee system but I guess it will be much easier if your child can be trained in the NHS if he/she plans to come back to the UK in longer term. If your child's plan is going to Australia after that, no qualifying exam will be required. If I were you, it will be the detrimental factor to consider.
Teaching quality - one of my besties was educated in HKU and qualified as a doctor after her trainee (houseman) years. Teaching was good, problem based learning with nice professors.
The 40% foreign student mix - it is no secret that it is easier to get into HK medics with "Non-HK exam" result, that 40% comprises of a very high proportion of Chinese descendants with a foreign (non-HK) passport. Nevertheless, their English are good, communicating within the university circle should not be a big problem.
The biggest hurdle - the houseman year (equivalent to trainee in UK?)in HK public health system is very very tough. Lots of pressure, not only from peers/seniors but also from experienced nurses. Trainees are treated as cheap labor in the hospital, and treated with little respects as trainees are not qualified doctors (due to the fact that they can do nothing other research without completing this qualifying year, I think the same goes with other industry anyways). They work extra long hours (long hours in HK means 70hrs per week). If anyone gets allocated to a very busy ward, be prepared for prolonged sleep deprivation. Lots of administrative burdens after "official" hours. Lots of gossip about the trainees over the slightest of things and not job related (maybe your child can be excluded because he/she does not speak the language). It is not that uncommon for trainee doctors to suffer from depression. Your child may have an extra challenge due to language barrier as most patients only speak Cantonese. Those patients who are better educated (hence who understands English), usually opt for private care in HK.
The pressure in public hospitals has got worse in recent years due to many reasons. My friend only got circa 5mins to see a patient before she had reached her mental limit and left HK. Patients easily waited 1-2 hours for a booked appointment pre-Covid (I.e. booked 11am, seen at 1pm). The average waiting hours in HK A&E is around a double of NHS despite HK is already charging patients £15/visit- so you can imagine the shortage of staffs.
HK is now recruiting heavily from mainland China doctors to join the healthcare system, probably it is a plus for your child as there will be more doctors and possibly professors and students speaking mandarin. The same goes for patients, around 1/5 of HK population immigrates from mainland China in the past 20 years- they all speak mandarin. HK has also just lifted the restrictions that a qualified doctor must be able to speak Cantonese, so I guess your child will not be that of an odd one out due to these changes. And I hate to say it but HK is quite racist (esp the older generation)...if your child is white, he/she will be fine.
The biggest advantage is that doctors in HK have got a higher salary in general. And it has got a much more assessable private sector in HK, that one does not necessarily to be a top notch consultant to be recruited in a private hospital/ clinic.
English is not my first language so apologies if it's bit hard to read. Hope it helps.