Britain's ecosystem also hasn't been isolated and untouched for many many generations. Isolated islands like Australia have far more unique plants and animals that could be wiped out by an invasive species. It doesn't take much for one to gain a foothold to the point of being impossible to remove, either.

The odds of an apple seed crossing from the US into Canada without a human involved are astronomically higher than one getting to Australia, hence customs are far more diligent in looking for that sort of thing.

Since they're already on high alert, everything looks suspicious I suppose.

There's certainly not a tv show then that follows border agents around like in Aus/NZ.