UK has one real production line - as the Astutes are finished everyone slowly transits over to the new SSBN's - there aren't the people to build totally in parallel
even the US has problems with capacity - it would need an expensive and long-term commitment to up the pace and add more capacity. As the initial Virginia target is about reached you could transfer a few late Los Angeles/early Virginias to Australia and stretch the delivery of the later Virginias a few years without as serious impact on capability. After all you're not really losing the transferred boats from the strategic balance.