For 30 Billion dollars US they are better off building up the Royal Navys conventional Carrier and Amphibious Warfare capability.
Yes, still have a nuclear detterent, but have it within the RAF and the CVs and SSNs of the Navy.
The SSBNs are a huge expense, and the only thing they are good for is a nuclear response, which is very unlikely to ever be needed. You can still have a deterrent force with a mixed deployment in two services (RAF and RN).
The most likely scenarios facing the Royal Navy is conventional force projection far away from home waters, Middle East, Asia, Africa, ect.
Just my opinion.