I think it's nearer 36, with the 4 Polaris boats north of the border and the rest being attack boats, piled up in Devon.The US are still using the Virginias. Good luck trying to steal one off the USN.
Closest we would get, is basing some Virginias out here some of the time.
As for borrowing a Trafalgar or LA class.. They would have to be refueled, upgraded. then disposed of. Any lease would consume existing submarine crew, and have to be based out of WA. Keep in mind the UK has never fully decommissioned and recylced any nuclear submarine. 27 are awaiting disposal. While the UK has many things to be envious of, their recycling of nuclear submarines is not one of them.
Failure puts UK’s reputation as responsible nuclear power at risk, auditors saywww.theguardian.com
Australia should start planning for something, as a new class is far away as it ever was, while the existing Collins need extensive refits and life extensions. Will see their availability rates drop as they go into life extension.
Maybe we look at things like additional P8's or MH60R's.
It's been a political football kicked up the road for decades - the US has a far more proactive approach as far as I can tell, possibly due to the sheer scale of material involved.
As to borrowing a T boat -please don't, they're all high mileage examples and maintenance hogs