It's the willingness and ability to take effective measures, not the geography, that matters.
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.
Here, nearby in NSW our public health system is really good. We're currently dealing with out third significant outbreak - seems to be an escape from an overseas arrival - but we can justifiably be confident that we are able to stamp these things out without too much disruption (relatively speaking).
(One of the world's biggest airports, several of Europe's busiest, frequent direct rail connection to the continent, frequent ferries on many routes, fully integrated free trade + work zone.)
"According to Getlink, the company which operates the Channel Tunnel, 1.64 million trucks traveled between the UK and the EU using the crossing last year. More than 2.5 million cars made the crossing, as did 51,229 buses and 2,012 freight trains.
Added to the even greater volumes of truck crossings made on ferries — the Port of Dover handled a record 2.6 million trucks in 2017, according to its own figures — it accounts for a massive annual volume."
People love to make excuses or put up obstructions to an idea, but solutions are almost always achievable with a little planning, resources and the will to make it happen.