"So new-build Victors and Buccaneers capable of launching long-range nuclear cruise missiles seems perfectly viable to me, and whilst we're at it why not new build Nimrods for our MPA?!!"
Because the designs date back to the 1950s using machinery, technology and industrial techniques long obsolete and out of date and no longer available. We'd have to rebuild from scratch a bespoke industrial base to create a design using 1950s kit and then shoehorn in 2010s technology and hope it works.
Nimrod is a classic lesson in why rebuilds from scratch will always go badly badly wrong.