As has been said already, opening up is not the problem per se, the problem is how many hospitalisations deaths, long-term or even permanent disability etc. will those cases translate into.
The UK's approach is not entirely wrong - indeed, it is mostly right in principle... it's just the execution and timing that is being botched. Lockdown the country to deeply suppress (or even eliminate) cases, vaccinate your population then re-open - that's what countries need to do.
Australia have done the first bit, but haven't done the second bit... so re-opening remains out of the question for them.
The US and UK have done the second bit, but still not enough to avoid major problems upon re-opening. Another 6-8 weeks of vaccinations and restrictions (tougher than what we have now) would be highly advisable IMHO.
The UK is currently seeing a drop in the case to hospitalisation ratio of around 2/3rds... and a huge drop in deaths per case. That is excellent and very welcome... but it isn't enough.
We also need to suppress infections for a host of other important reasons, even not considering how many hospitalisations they may result in, but preferably without lockdowns. That means light-touch measures like mask mandates, free and readily available testing, track and trace etc. and, of course, a continued fast rollout and high uptake of the vaccines.
Australia and anywhere else that has struggled to rollout the vaccine(s) have a long road ahead, unfortunately - but, they have done a great job thus far and just need to get their act together better on their vaccine rollout. The UK had more new cases yesterday than Australia has ever had. Through their suppression strategy, they bought themselves time to get vaccinations done, but alas it looks like they may have largely squandered that time, but I'd argue they're still in a far better position than us.