Apart from some ridiculously pompous and chauvinistic comments below, there is a total lack of the infrastructure, training and other problems that "going nuclear" involves.
From the vast civil works needed to provide such things as covered building facilities, ship lift, secure overhaul berth(s), to training facilities (possibly including small land reactor (*), may now be replaced with simulator)
And, without a civil nuclear programme, who/where will provide such services as nuclear fuel reprocessing - less of a problem nowadays with lifetime cores. Also things such as back-up/specialist servicing skills and even jobs for after the nuclear engineers leave the service ? A lot of these costs are "shared" or hidden in the civil nuclear industry.
(*) The original RN nuclear reactor was at the old Royal Naval College in East Central London !
LET ALONE, where will the boats be built and operated from ? Are there reasonably secure sea lanes to the open ocean, and how will the guard launches and ships operate.
LET ALONE the political ramifications. Will the Australian nuclear sub base allow RN And USN subs dock ? Will they allow nuclear weapons ?
ETC., ETC., ETC,. ...