Budget cuts over years. The subs still exist, just can't go anywhere.
I don't know if you saw the post in a previous bread but somebody decided to "doom and gloom" about how Iranian drones could strike Portsmouth (the original one) and do a "British Pearl Harbor." Considering the state of the ships there I'd say it would be a mercy killing. I don't recall finding the actual name but there is a Type 45 destroyer at that base referred to as "HMS Doomed" these days