Australia has a lower population than California, by the way. Obviously, its pop is mostly spread over the east coast, but compared to the UK it's extremely sparse, with no tunnels connecting it to the mainland. It's disingenuous (at best) to think that the island nations that had successful lockdowns were all doing something magically better than other places with significant restrictions to daily life like France, Italy, California, etc. because they couldn't realistically close their borders to ground traffic.

And obviously you can close a land border as well, it's just a matter of whether you're willing to do it. Australia also imposed tight restrictions on most of the land borders between states.