Here's an interesting geological fact about New Zealand and Australia: they're both islands! Believe it or not, it's much easier to control population flow into an island, than it is over a land border.
If "it's much easier to control population flow into an island" then you still have to step up and control it. The UK government did not even show interest in doing that.
The UK government mismanaged their borders, and most other aspects of the pandemic; notable exception being vaccination.
Not really, Australia is an continent not an island.
And to be precise, only Europe and Asia are connected by land borders (suez canal, panama canal).
More, UK is an island, also Ireland, Japan... and they don't have much less cases then the rest of the world