What tends to get overlooked in discussions concerning imported carp is that carp are not an indigenous UK species. If we hadn't imported them, we wouldn't have them - it's as simple as that.
As to their country of origin, that's purely academic, surely? Providing they have the all-clear so far as their health is concerned, I don't see that it makes much difference where they come from.
Not that I think we need get overly excited about fish coming from Australia - transport costs will preclude it as a viable option.
The worry, as always, will be illegal carp imports from continental Europe, with all the accompaying implications for the spread of disease and parasites.