Sure, the UK and Australia can't pressure China effectively, but China can't really pressure the UK (maybe it can Australia rather more) in return. There is basically nothing essential China can make that UK or Aus can't go through a middleman (e.g. Singapore) for, or acquire elsewhere. It's inconvenient, but the cost will be so small compared to the wider economy as to barely matter.The trade is still a sizeable amount and will only increase if China's economy will also prioritize domestic growth instead of only exports. Also indirectly China supplies the bulk of industrial raw ingredients necessary for transshipment of half fabricates which is important for U.K.'s international trade. Probably a larger chunk of their export than marmite and cheddar. So they are very vulnerable on the supply side as well. Trade with the E.U. is also likely to decrease if they close the border for the simple fact paperwork and cumbersome customs processes will slow it down. Without an economic block it is simply impossibe to make unilateral trade deals with China that would make any kind of demands on the regime. Even the U.S. can't change China's policy much in terms of IP protection, access to Chinese markets and unfair competition due to state subsidies. Even tariffs on Chinese steel or whatever won't put a dent in the trade deficit due to the huge demand on Chinese production and supply.
The UK's delusions about trade deals with anyone at all are a moot point - the government has deceived the population about all manner of things over Brexit and the country will pay the cost in due course. But it can feel quite free to move trade away from China to other places. There are no shortage of countries happy to make steel or widgets for the UK if China declines to with opportunities to invest in in return.
China is obviously in the process of shutting down HK autonomy and integrating it thoroughly into the Chinese system. We could never imagine the CCP would permit an outpost of free capitalist democracy in its country's borders indefinitely, with the inherent challenge that presents. It's only taken this long because China needed to be strong enough to do it without too serious ramifications. HK used to be about 15-20% of China's economy, and it's now ~5%. That's a year's GDP growth, easy to suck up a spot of loss whilst pounding it into line.HK's special status is beneficial for China as well because of it's independent legal process enabling the settlement of disputes through neutral insurance companies. As a guarantee for international businesses to invest in China this is pivotal and the regime knows this which is why they have treated the HK protests with comparative silk gloves. They even went so far as to concede with the extradition treaty but the initial demands didn't suffice anymore for the protestors. Ofcourse China won't allow a provocation from Australia over this or allow the protestors for China to lose face. It needs both internal support and the trust from investors so it's a difficult balance. I guess technically Shanghai could take over HK as China's international trading hub but it's something they'll try to avoid at all costs.
It is simply bizarre to think this was not always coming. Look at what China has done in Tibet, or to Xinjiang. Damn right it's kids gloves stuff in comparison, but no-one can be under any illusion that HK is to increasingly know that Beijing is boss. What China wants is to hammer HK into line whilst retaining all the advantages that HK held compared to the mainland. The rest of the world should rightfully tell it to get lost.