Virginia and Astute both use a non-refuel requirement reactor - there's ways and means that could be used to get around all those restrictions - go native/license build on the boat and lease the core which would be built in the US, the Australians put the rest of it together offer the reactor compartment up much as they do in the yard in the US, run it til EOL and then return the core for recycling. Treat the core as a lightbulb fitting, screw it in, unscrew when it goes out.The submarine problem is a whole lot more than buying them from the UK or the US. For starters does Australia have the capability today to process the fuel? To dispose of the fuel? To operate the nuclear power plant safely? Frankly, you have to have a nuclear power industry to do all of the above and with much expenses than buying a submarine. Then there are the political realities, Australia will want to build their subs in Australia.
Technically, possible. Politically, not.
Rob's correct in eliminating the UK from consideration - the Astute needed a lot of US input to get it over the line - the yard is back up to speed and doing good work again, and in a good place to deliver Successor but if I were shopping around, and were Australian, I'd have the US on speed dial.